Geomorphic effects of large wood jams on a sub‐antarctic mountain stream

Dead wood pieces, especially when organized in jams, play an important geomorphic role in streams because of the effects on flow hydraulics, pool formation and sediments storage. The increase in stream morphological diversity and complexity also exerts an important ecological role. This work reports on geomorphic role of large wood (LW) pieces and jams in a third‐order mountain stream located in the Southern Tierra del Fuego (Argentina), and draining an old‐growth Nothofagus‐forested basin not influenced by the beavers damming activity. Even if the in‐stream number of wood pieces (length >1 m; diameter >0.1 m) is comparable to that observed in other climatic areas, the slow growth of the Nothofagus forest causes a lower wood abundance in terms of volumetric load. Because of the relatively small dimensions of the surveyed LW pieces, almost 70% of them demonstrated to have been fluvial transported and also the wood jams reflect the apparent dynamic nature of wood in the channel. Wood jams exert a significant influence on the channel morphology, representing almost half of the drop caused by steps and being responsible for the creation of 30% of the pools. LW‐forced pool volume is strongly and positively correlated to the height of the LW jam. The geomorphic influence of LW jams is also exerted by a considerable sediment storing capacity. Copyright © 2008 John Wiley & Sons, Ltd.     

Evaluating the Effects of Culvert Designs on Ecosystem Processes in Northern Wisconsin Streams

Culvert replacements are commonly undertaken to restore aquatic organism passage and stream hydrologic and geomorphic conditions, but their effects on ecosystem processes are rarely quantified. The objective of this study was to investigate the effects of two culvert replacement designs on stream ecosystem processes. The stream simulation design, where culverts accommodate bankfull width and streambeds are reconstructed through the culvert, was compared with the bankfull and backwater design, where streambeds were left to fill naturally, as well as to non‐replaced culverts. We predicted that stream simulation culverts would best preserve water velocity and coarse particulate organic matter (CPOM) retention within the culvert relative to upstream reaches, and that both replaced culvert styles would exhibit rates closer to upstream reaches than non‐replaced culverts. In addition, we predicted that ecosystem processes (CPOM retention, transient storage and nutrient uptake) would be similar in reaches upstream and downstream of both replaced culvert styles, because both designs are constructed to maintain stream slopes and bankfull widths through the structure. We found that stream simulation design better maintained CPOM retention through culverts compared with non‐replaced and bankfull and backwater design culverts, but observed no differences in ecosystem processes between reaches located upstream or downstream of replaced culverts. Although the stream simulation design requires additional streambed construction relative to the bankfull and backwater design, this step may lead to additional improvement if maintaining ecological conditions through the culvert is an important restoration goal. Copyright © 2017 John Wiley & Sons, Ltd.     

LARGE‐WOOD LOADING FROM NATURAL AND ENGINEERED PROCESSES AT THE WATERSHED SCALE

Large wood, both live and dead, is essential for producing complex habitat in many streams, especially in forested watersheds that support salmonid populations. The addition of engineered wood structures is a common approach taken in many streams where past watershed management practices have resulted in reduced wood loading. We examined six 300‐m stream reaches in the Lagunitas Creek watershed, Northern California, to determine (i) the distribution of large wood in the bankfull channel and 10‐year floodplain, (ii) the influence of large wood on the size and distribution of pools and (iii) whether streams with engineered wood structures had greater diversity of pool habitat to support salmonid populations. We found that the amount of large wood in the bankfull channel and the amount available for recruitment from the 10‐year floodplain were highly variable among and within reaches examined and largely dependent on the local geomorphic setting. Stream reaches with engineered wood structures had elevated pool frequencies relative to reaches without these structures, suggesting a higher capacity to support salmonids during critical life stages. Among large wood pieces that had a strong influence on pool formation, 23% had an attached root wad and 66% were part of a cluster. All of the study reaches we examined had lower volumes of large wood in their bankfull channels than similar stream types with natural wood‐loading levels, suggesting that increased additions of large wood could provide ecosystem benefits over time. These principles can be understood and transferred effectively to other watersheds using a framework of wood‐loading process domains. Copyright © 2012 John Wiley & Sons, Ltd.     

Estimating channel‐forming discharge in urban watercourses

Several methods of estimating channel‐forming discharge were conducted on 12 quasi‐stable urban stream channels ranging from 9 to 99% urban land use to test their applicability in the urban condition. Bankfull stage was identified at a series of locations along each study reach and it was found that the most consistent observations of bankfull discharge occurred during flood conditions where bankfull stage was identified at the top of point bars along the convex arc of bends. The largest errors in estimation occurred at gauge stations where cross‐sectional geometry had been altered to conform to bridges or culverts rather than the channel morphology. Independent evaluations of channel forming discharge were conducted by 11 practitioners ranging from 10 years to 43 years of experience with similar findings and errors. Various methods of relating frequency return periods were evaluated using annual peak series discharge observations and continuous 15‐min systematic discharge records using partial duration series analysis. Bankfull discharge was observed to occur more than once a year in all of the urban streams studied and often averaged from 4 to 8 bankfull discharge or larger events per year. In one particular case in a single given year 18 events exceeding bankfull discharge were observed. No specific correlations were identified between frequency return periods and land use change. However, based upon the findings of this study, the applicability of employing annual series peak discharge data to evaluate bankfull frequency return in urban stream channels is highly discouraged. Copyright © 2010 John Wiley & Sons, Ltd.     

CONTROLS ON THE LONGITUDINAL DISTRIBUTION OF CHANNEL‐SPANNING LOGJAMS IN THE COLORADO FRONT RANGE,USA

Channel‐spanning logjams completely span the active channel and create longitudinal discontinuities of the water surface and stream bed across at least two‐thirds of the channel width. These jams disproportionately affect channel process and form relative to smaller jams that do not span the entire channel width. We analyze a spatially extensive dataset of 859 channel‐spanning jams distributed along 124 km of 16 distinct rivers on the eastern side of Rocky Mountain National Park, Colorado, USA, with drainage areas spanning 2.6 to 258 km² and diverse valley geometry and forest stand age. We categorized valley geometry in terms of lateral confinement (confined, partly confined, or unconfined), which correlates with gradient. Jams exhibit substantial downstream variability in spacing at channel lengths of 10²–10³ m. The number of jams within a reach is explained by a statistical model that includes drainage area, valley type (lateral confinement), and channel width. Longitudinal spacing of jams drops substantially at drainage areas greater than ~20 km², although jam spacing exhibits tremendous variability at smaller drainage areas. We interpret the lack of jams at larger drainage areas to reflect increasing transport capacity for instream wood. We interpret the variability in jam spacing at small drainage areas to reflect local controls of valley geometry and associated wood recruitment and fluvial transport capacity. Our results suggest that management of instream wood designed to facilitate the formation of channel‐spanning jams can be most effectively focused on smaller drainage areas where these jams are most abundant in the absence of management that alters instream wood recruitment or retention. Unmanaged streams in the study region with drainage area <60 km² have ~1.1 channel‐spanning jams per 100 m length of stream. The cumulative effects of these jams on instream storage of sediment and organic matter, hyporheic exchange, instream habitat, stream metabolism, and channel–floodplain connectivity are likely to be enormous. Copyright © 2012 John Wiley & Sons, Ltd.     

Mass Dispersal of Terrestrial Organisms During First Flush Events in a Temporary Stream

Temporary streams expand and contract seasonally, forming a complex mosaic of aquatic, amphibic and terrestrial habitats. We studied the terrestrial arthropod fauna at the surface of the dry river bed as well as the fauna of Coarse Particulate Organic Matter (CPOM) deposits 0, 5 and 10 days after first flush events (years 2004–2006) along the Pardiela stream (SE Portugal). During the dry period, large amounts of organic material accumulated at the surface of the dry bed, colonized by abundant terrestrial arthropods (mean density: 13.3 ± 15.29 Ind g DM (Dry Mass of CPOM)). Arthropod density peaked in fresh flood deposits (mean density: 35.8 ± 33.4 Ind g DM), and subsequently decreased within time. Concurrently, the relative composition of the arthropod community changed from Day 0 to Day 10. The present results demonstrated that the dry bed of temporary streams served as a major habitat for terrestrial arthropods. During the first flush events, a mass dispersal of terrestrial arthropods, rafting on floating CPOM, occurred, subsequently forming distinct deposits along the channel margin. These deposits may constitute critical habitats, refugia and food resources for local and regional terrestrial arthropod assemblages. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of urbanization on stream hydraulics

Urbanization results in major changes to stream morphology and hydrology with the latter often cited as a primary stressor of urban stream ecosystems. These modifications unequivocally alter stream hydraulics, but little is known about such impacts. Hydraulic changes due to urbanization were demonstrated using two‐dimensional hydrodynamic model simulations, comparing urban and non‐urban stream reaches. We investigated three ecologically relevant hydraulic characteristics: bed mobilization, retentive habitat, and floodplain inundation, using hydraulic metrics bed shear stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area. We hypothesized that urbanization would substantially increase bed mobilization, decrease retentive habitat, and due to increased channel size would decrease floodplain inundation. Relative percent area of bed disturbance was 4 times higher, compared with that of the non‐urban stream at bankfull discharge. SSWH availability rapidly diminished in the urban stream as discharge increased, with SSWH area and patch size 2 times smaller than the non‐urban stream for a frequently occurring flow 0.7 times bankfull discharge. Floodplain inundation decreased in frequency and duration. These results demonstrate changes in hydraulics due to urbanization that may impact on physical habitat in streams. New “water sensitive” approaches to stormwater management could be enhanced by specification of hydraulic regimes capable of supporting healthy stream habitats. We propose that a complete management approach should include the goals of restoration and protection of natural hydraulic processes, particularly those that support ecological and geomorphic functioning of streams.     

Quasi‐equilibrium conditions of urban gravel‐bed stream channels in southern Ontario,Canada

Urban gravel‐bed stream channels in southern Ontario, Canada identified to be in a state of quasi‐equilibrium have been studied over the past 15 years and compared against rural gravel bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1‐year return period with many sites ranging between 2 and 18 bankfull events per year with higher intensity and shorter duration urban flood responses, which further identified significant limitations in using annual peak discharge methods for predicting morphological forming flows in urban watersheds. The cumulative volume of bankfull and larger flood events from the urban stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed material supply was found to decrease with increasing urbanization and the reduction in bed material supply appears to be off‐set by the smaller bankfull channel width, depth and access to floodplains during large flood events. Field evidence may also suggest a even greater reduction in channel width trajectory, relative to the rural setting, with expansive floodplains to maintain quasi‐equilibrium conditions as bed material supply continues to decrease with increased anthropogenic activity. Copyright © 2010 John Wiley & Sons, Ltd.     

Geomorphic applications of stream‐gage information

In the United States, several thousand stream gages provide what typically is the only source of continuous, long‐term streamflow and channel‐geometry information for the locations being monitored. In this paper, the geomorphic content of stream‐gage information, previous and potential applications of stream‐gage information in fluvial geomorphic research and various possible limitations are described. Documented applications include studies of hydraulic geometry, channel bankfull characteristics, sediment transport and channel geomorphic response to various types of disturbance. Potential applications include studies to determine the geomorphic effectiveness of large floods and in‐stream habitat change in response to disturbance. For certain applications, various spatial, temporal and data limitations may render the stream‐gage information of limited use; however, such information often is of considerable value to enable or enhance geomorphic investigations. Published in 2008 by John Wiley & Sons, Ltd.     

Large wood aids spawning Chinook salmon (Oncorhynchus tshawytscha) in marginal habitat on a regulated river in California

To determine whether large wood (LW, ≥1‐m length, ≥10‐cm diameter) plays a role in Chinook salmon (Oncorhynchus tshawytscha) redd (i.e. egg nest) placements in a regulated, Mediterranean‐climate, medium‐sized river (where channel width is less than the upper quartile of length of potential instream wood pieces), characteristics of 527 large wood pieces, locations of 650 redds, and mesohabitat delineations (riffle, run, glide, pool) were collected during a spawning season along a 7.7 km reach directly below Camanche Dam on the Mokelumne River, CA. LW was regularly distributed across the study reach an average 70 LW pieces km^‐1. Some LW clustering was evident at islands and meander bends. Spawners built 85% of redds within one average channel width (31 m) of LW. Spawners utilized LW within a 10 m radius 36% of the time in the upper 3 km rehabilitated reach, and 44% of the time in the lower 4.7 km marginal habitat reach. A greater percentage of LW was utilized in riffles in the upper 3 km reach where 90% of redds were built, while a larger percentage of spawners used LW in riffles in the lower 4.7 km reach. LW‐redd interactions occurred at greater rates than by random chance alone in the lower 4.7 km reach, which implies that LW aids spawning in marginal habitats. River managers and salmonid spawning habitat rehabilitation (SHR) projects should take LW additions into consideration as an important component of river rehabilitation. Copyright © 2010 John Wiley & Sons, Ltd.     

A LIDAR‐DERIVED EVALUATION OF WATERSHED‐SCALE LARGE WOODY DEBRIS SOURCES AND RECRUITMENT MECHANISMS: COASTAL MAINE,USA

In‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions. Copyright © 2011 John Wiley & Sons, Ltd.     

The development of hydraulic and geomorphic complexity in recently formed streams in Glacier Bay National Park,Alaska

Geomorphic and hydraulic complexity within five streams representing 200 years of stream development were examined in Glacier Bay National Park, Alaska. Channel geomorphic units (CGUs) were mapped using a hierarchical approach, which defined stream habitat according to morphological and hydraulic characteristics. Detailed hydraulic assessment within the geomorphic units allowed differences in hydraulic characteristics across the 200‐year chronosequence to be documented. Channel geomorphology and hydrology changed as stream age increased. Younger streams were dominated by fast flowing geomorphic units such as rapids and riffles with little hydraulic or landscape diversity. As stream age increased, slower flowing habitat units such as glides and pools became more dominant, resulting in increased geomorphic, hydraulic and landscape diversity. These results suggest that geomorphic and hydraulic complexity develop over time, creating habitat features likely to be favoured by instream biota, enhancing biodiversity and abundance. Copyright © 2009 John Wiley & Sons, Ltd.     

Spatial distribution of large wood jams in streams related to stream‐valley geomorphology and forest age in Northern Michigan

Geomorphology at the scale of stream valleys influences smaller scale processes that give rise to spatially distributed patches, including large wood jams (LWJ) in streams. Understanding the spatial distribution of LWJ along streams with reference to large‐scale geomorphology is valuable for understanding stream and riparian interactions, and may be critical for effective stream management and restoration. We surveyed the locations of LWJ along 18 stream segments within study areas defined by stream‐valley geomorphology. The objective of this study was to test the prediction that LWJ in this system will be aggregated in areas defined by stream‐valley geomorphology, but be randomly distributed at smaller scales. The spatial distribution of LWJ was analysed by a one‐dimensional K‐function analysis capable of detecting aggregated, random and segregated patterns at different scales. The prediction that LWJ aggregate in areas defined by stream‐valley geomorphology was supported: LWJ aggregated at scales up to several kilometres in three streams. LWJ also was segregated at smaller scales in two of these streams; this was detectable when several stream valley segments were considered together. The prediction that LWJ would be randomly distributed at smaller scales was supported at most smaller scales for most streams. In fact, 40% of individual stream valley segments contained LWJ that were randomly distributed at all scales. Twenty‐seven per cent of individual stream valley segments showed segregated LWJ distributions. Large‐scale aggregation of LWJ evidences the need to select reference reaches that encompass several geomorphic patches at the scale of the stream valley. Segregated patterns of LWJ distributions evidence opportunities to better understand the relationships between hydraulic systems, material transport dynamics and riparian forests. Copyright © 2009 John Wiley & Sons, Ltd.     

Post‐flooding distribution and characteristics of large woody debris piles along the semi‐arid Sabie River,South Africa

The formation of large woody debris (LWD) piles has a profound impact on channel patterns and riparian succession in temperate rivers. The opportunity to study LWD along the Sabie River, a river in the semi‐arid region of Kruger National Park, South Africa, arose in February 2000 after a significant flood (c. 100‐year return interval) removed a large proportion of the fully mature riparian forest and other plant communities. Much of the uprooted vegetation was deposited as LWD piles (woody vegetation accumulations deposited on the ground > 0.1 m³) throughout the riparian and upland zones. In this article we describe the spatial distribution patterns of LWD as related to geomorphic channel type and flood frequency zone, and assess pile composition characteristics six months after the flood. Within the areas surveyed there were 68 LWD piles per hectare, the median size of LWD piles was 4.6 m³ but pile sizes (by volume) varied widely. Pool/rapid geomorphic channel types had the highest density of LWD piles (79 ha^?1) and the largest piles (by volume) were in the bedrock anastomosing channels (mean = 124 m³). Piles were larger in the seasonal and ephemeral flood frequency zones (mean = 54 m³ and 55 m³) than piles in the active zone (c. 2 m³). The patterns of distribution and volume of LWD will affect the subsequent development of vegetation communities as debris piles form a mosaic of patches of surviving organisms and propagules that can strongly influence the initial trajectory of succession. The amount, distribution, and subsequent decomposition of LWD are different from that reported for temperate rivers, suggesting that the role of LWD may be different on non‐floodplain rivers such as the Sabie in semi‐arid South Africa. Copyright © 2004 John Wiley & Sons, Ltd.     

Quantifying Fish Habitat Associated with Stream Simulation Design Culverts in Northern Wisconsin

This study investigated the effects of culvert replacement design on fish habitat and fish weight by comparing substrate diversity and weight at three stream simulation (SS)‐design and three bankfull and backwater (BB)‐design sites on the Chequamegon‐Nicolet National Forest, Wisconsin. Stream channel cross‐sections, Wolman substrate particle counts, and single‐pass backpack electro‐fishing survey data were used to quantify fish habitat and fish weight in 50‐m upstream and downstream sample reaches at each site. We applied generalized linear mixed models to test the hypothesis that substrate size and fish weight did not differ according to stream‐crossing design type (SS or BB) and location (upstream or downstream). Substrate particle sizes were significantly greater upstream of the stream crossing when compared to downstream of the stream crossing at both SS and BB sites for riffles and pools. Substrate particle sizes were also significantly greater upstream of BB sites when compared to upstream of SS sites. Results of this study indicated statistically greater individual fish weights upstream of SS‐design sites in comparison to upstream of BB‐design sites in first‐ to third‐order low gradient streams. These results suggested that the SS‐design approach appears to be more effective at transporting sediment downstream, and illustrated the value of using fish weight as an indicator of biological success for stream‐crossing designs. Published 2017. This article is a U.S. Government work and is in the public domain in the USA. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.     

Detecting Fluvial Wood in Forested Watersheds using LiDAR Data: A Methodological Assessment

It has long been known that large wood in rivers increases channel complexity and is a primary driver of geomorphic change in forested mountain streams in the Pacific Northwest. Studies analyzing the presence and distribution of fluvial wood are often limited in their spatial extents to the site or reach scales because of the intensive fieldwork required for comprehensive wood surveys. Remote sensing techniques are beginning to allow researchers to assess fluvial wood dynamics and distributions on a basin or regional scale. We used 2009 high‐resolution light detection and ranging (LiDAR) point cloud data to detect and quantify wood within five forested watersheds in the Oregon Coast Range. We filtered the LiDAR data to remove the forest canopy over the stream channel and visually inventoried fluvial wood based on its distinct shape within the channels. We derived several wood and stream morphometric variables to test theories relating to wood abundance and positioning in the lower reaches of streams. We were able to detect fluvial wood with confidence; however, validation of results with ground‐truth data was difficult in the study due to the dynamic and mobile nature of wood through time. We mapped a total of 163 single logs and 55 logjams within the five study watersheds. We did not find statistically significant differences between individual pieces and jam positioning in relation to slope; however, the surveyed wood was often found in areas of lower stream power. This research shows that it is possible to use height‐filtered LiDAR to detect in‐stream wood in densely forested watersheds and has the potential to be employed in future wood studies across broad spatial scales. Copyright © 2015 John Wiley & Sons, Ltd.     

SPATIAL PATTERN,DENSITY AND CHARACTERISTICS OF LARGE WOOD IN CONNECTICUT STREAMS: IMPLICATIONS FOR STREAM RESTORATION PRIORITIES IN SOUTHERN NEW ENGLAND

Many streams have been modified so extensively that river managers do not have clear reference conditions to frame targets for stream restoration. Large woody debris (LWD) has long been recognized as an important influence on both geomorphic and ecologic processes in stream channels; however, there have been few studies of LWD dynamics in New England. Although this region is heavily forested today, the forest is predominantly young (70–90 years old) regrowth following a historical episode of severe deforestation. This study presents the results of an extensive census of LWD and associated stream characteristics in over 16 river kilometres of northeastern Connecticut streams and represents the first reported inventory of wood loading and sorting in Southern New England. Results of this study indicate that wood loading and jam frequencies in the study region are low: 2.5–17.8 and 0.5–5.51 per 100 m, respectively. Orientation of LWD is predominantly parallel to flow, an indication that these streams are not retaining organic matter or sediment, which has important geomorphic and ecologic implications. Results imply that stream recruitment of LWD is still lagging from the massive forest conversions of the 18th and 19th centuries. Given the low wood loadings observed in the study reaches, manual wood addition and continued forest regeneration would likely improve both habitat diversity and organic matter and fine sediment retention in these systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of large woody debris on surface structure and aquatic habitat in forested streams,southern interior British Columbia,Canada

It is well known that large woody debris (LWD) plays an important functional role in aquatic organisms' life. However, the influence of LWD on channel morphology and aquatic environments at watershed levels is still unclear. The relationships between wood and surface structure and aquatic habitat in 35 first through fifth order streams of southern interior British Columbia were investigated. Study streams in the channel networks of the study watersheds were classified into four size categories based on stream order and bankfull width: Stream size I: bankfull width was less than 3 m, Stream size II: 3–5 m, Stream size III: 5–7 m, Stream size IV: larger than 7 m. We found the number of functional pieces increased with stream size and wood surface area in stream sizes I, II and III (24, 28 and 25 m²/100 m², respectively) was significantly higher than that in stream size IV (12 m²/100 m²). The contribution of wood pieces to pool formation was 75% and 85% in stream sizes II and III, respectively, which was significantly higher than those in stream size I (50%) and size IV (25%). Between 21% and 25% of wood pieces were associated with storing sediment, and between 20% and 29% of pieces were involved in channel bank stability in all study streams. Due to long‐term interactions, LWD in the intermediate sized streams (Size II and III) exhibited much effect on channel surface structure and aquatic habitats in the studied watersheds. Copyright © 2008 John Wiley & Sons, Ltd.     

Stream temperature dynamics within a New Zealand glacierized river basin

Knowledge of river thermal dynamics for glacierized basins is limited, despite the high sensitivity of these systems to climatic change/variability. This study examined spatio‐temporal water column and streambed temperature dynamics within a New Zealand glacierized river basin over two melt seasons. Water column temperature was recorded at three sites along the mainstem channel and four hillslope/groundwater‐fed tributaries. Air temperature, precipitation and stream discharge were monitored to characterize hydroclimatological conditions. Streambed temperature was monitored at the upper and lower main river sites at 0.05, 0.2 and 0.4 m depth. Water column temperature rose on average 0.6°C km^?1 along the glacier‐fed mainstem. Temperature was elevated during warmer periods but the downstream increase was reduced due to greater meltwater production (consequently a larger total stream flow volume for atmospheric heating) plus a proportional reduction in warmer groundwater contributions. Hillslope/groundwater‐fed tributaries yielded a range of temperature patterns, indicating variable sourcing (meltwater or rainfall) and residence times. In the upper basin, streambed temperature was warmer than the water column, suggesting groundwater upwelling; however, during high runoff events, water column and streambed temperature converged, indicating downwelling/heat advection by channel water. At the lower site, streambed temperature mirrored the water column, suggesting greater surface water/atmospheric influences. Key drivers of stream thermal regime were: (1) relative water source contributions, (2) prevailing hydroclimatological conditions, (3) distance from source, (4) total stream flow volume and (5) basin factors (specifically, valley/channel geomorphology and riparian forest). High magnitude precipitation events produced a contrasting stream thermal response to that reported elsewhere. In contrast to European alpine research, streams showed a reduced thermal range owing to the relatively mild, wet melt season climate. This New Zealand case study highlights the importance of understanding basin‐specific modification of energy and hydrological fluxes for accurate prediction of stream thermal dynamics/habitat and ecological response to climatic variability and change. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessing channel geometry in response to land use disturbance in a low-relief,glacially conditioned setting

Bankfull stage, the highest flow elevation contained by a river channel before overbank flooding occurs, is the presumed threshold for channel morphological change. At bankfull, the channel boundary experiences the highest shear stress, producing somewhat predictable hydraulic relationships and a cross-sectional form. However, land use and glacial conditioning can profoundly impact a channel's geomorphic responses. Two common methods characterize bankfull flow: one based on flow frequency and other based on channel form. This study considers a simplified approach to identify upstream land use relative to estimates of bankfull flow versus a channel's geometric form. The approach compares archived geomorphic surveys of 140 river reaches in southern Ontario, Canada, to 2-year flood quantiles modelled from historical flood data of 207 gauge stations. Flood frequency analysis determines that annual maximum series (AMS) datasets, fitted to optimized probabilistic distributions, underestimate discharge for low-magnitude, high-frequency flood events compared to partial duration series (PDS) datasets. For smaller drainage areas (<100 km²) associated with an extensive agricultural activity and/or urbanization, the estimates of bankfull discharge (Q_bf) generated by cross-sectional channel geometry are greater than the gauge-derived Q₂ values. Channels impacted by high levels of upstream land use disturbance show statistically significant lower width-to-depth ratios (p < 0.001) and a trend towards a finer D₅₀ bed material, suggesting enhanced surface runoff delivering more mobile finer sediments and channel confinement or incision. This research quantifies the formative bankfull stage to better understand the link between land use and a channel's ‘natural’ hydrogeomorphic response in a low-relief, glacially conditioned setting.