Using wavelet analysis to detect changes in water temperature regimes at multiple scales: effects of multi‐purpose dams in the Willamette River basin

Maintaining the natural complexity of water temperature regimes is a key to maintaining diverse biological communities. Insect communities, food webs, and fish respond to the magnitude and duration of water temperature fluctuations. Disruption of these natural patterns has the potential to alter physiological processes, behavioural adaptations, and community structure and dynamics. We analysed multiple >300‐day time series of water temperature from the Willamette River basin, Oregon, to assess the impact of large multi‐purpose dams on water temperature variability at temporal scales ranging from 1 to 32 days, short temporal scales that are commonly ignored. We applied wavelet analysis to quantify the variability of water temperature at multiple temporal scales simultaneously. We compared water temperature regimes above and below dams and before and after dam construction. The advantages of wavelet analysis are the ability to examine all temporal scales simultaneously and independently as well as the ability to preserve the temporal context of the wavelet coefficients. We were able to detect significant (p < 0.0001) reductions in water temperature variability, defined as the variability of the wavelet coefficients, as a result of dams at the 1‐, 2‐, 4‐, and 8‐day scales. There were no significant differences in water temperature variability between managed and natural flows at the 16‐ and 32‐day scale (p = 0.80). In addition to the well‐documented effects of dams on seasonal patterns in water temperature or on water temperature extremes, our results demonstrate that dams have significantly muted the small temporal scale variance in water temperature patterns to which many organisms may have been adapted. Conserving or restoring natural temperature patterns in rivers will require attention to these small‐scale complexities. Published in 2007 by John Wiley & Sons, Ltd.     

Influence of Hydrological Connectivity on Plankton Communities in Natural and Reconstructed Side‐Arms of a Large New Zealand River

We sampled natural and reconstructed side‐arms during different stages of hydrological connectivity with a large floodplain river in northern New Zealand, to determine whether re‐establishment of connectivity would be an effective strategy for restoring plankton communities in former side‐arms. Connectivity between side‐arms and the river was moderated by water level and influenced flow rates and closure of inlets and outlets. Physicochemical conditions were more strongly related to the connectivity phase than to habitat type (river, natural or reconstructed side‐arm), except during low connectivity when natural side‐arms in particular were characterised by higher ammonium (NH₄‐N) and total phosphorus (P) concentrations, as well as specific conductivity. Dissolved reactive phosphorus (PO₄‐P), water temperature, conductivity and dissolved oxygen were identified as explanatory variables of phytoplankton and zooplankton community composition, which along with total nitrogen (phytoplankton) or total suspended solids (zooplankton) explained 44–52% of variation. Phytoplankton community composition and the abundance of several dominant or discriminatory taxa were affected by connectivity but not habitat type, whereas habitat and connectivity both had significant effects on zooplankton communities and abundances of the cladoceran Bosmina meridionalis. Significant interactions between connectivity and paired habitat types occurred for abundances of the diatom Asterionella, the cryptophyte Cryptomonas, the rotifer Synchaeta oblonga and cyclopoid copepods, reflecting differential responses to connectivity among habitats by these taxa. Overall, these results underscore the importance of hydrological connectivity between side‐arms and rivers in moderating plankton community composition, and highlight unpredictable trajectories of community development and alternative transient states that can occur soon after side‐arm reconnection. Copyright © 2016 John Wiley & Sons, Ltd.     

A 12‐month sediment and nutrient budget in a floodplain reach of the River Adour,southwest France

The sediment and nutrient budget in the floodplain reach of the River Adour in southwest France has been examined over a 1‐year period (1991–1992). Outputs of total suspended matter, dissolved inorganic nitrogen (i.e. nitrate+ammonium ions) and dissolved inorganic phosphorus (DIP) and particulate phosphorus (PP) have been compared with inputs. Alternating phases of mobilization and retention are observed for all the elements considered. Low water periods in summer allowed retention of both soluble material (24–26% for inorganic nitrogen) and suspended matter (9–19%). However, mobilization of PP associated with suspended matter transport during these periods. Elevated discharges associated with increased rainfall during the autumn period induced significant mobilization for all elements. In contrast, the winter flood event gave rise to a retention of particulate material (i.e. suspended matter and PP, 11% and 26%, respectively), but a loss of soluble materials (5% for inorganic nitrogen and 186% for DIP). The spring flood event produced retention for particulate materials (26%) while inducing losses of inorganic nitrogen (9%) and gains of DIP (7.4%). These results are related to riparian vegetation and the dynamics of nutrient cycling within the floodplain. Copyright © 2000 John Wiley & Sons, Ltd.     

Seasonal Rearing Habitat in a Large Mediterranean‐Climate River: Management Implications at the Southern Extent of Pacific Salmon (Oncorhynchus spp.)

Pacific salmon (Oncorhynchus) use a variety of rearing environments prior to seaward migration, yet large river habitats and their use have not been well defined, particularly at the southernmost salmon range where major landscape‐level alterations have occurred. We explored juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) presence along the river continuum and in main‐channel and off‐channel habitats of a regulated California Mediterranean‐climate river. Over an 8‐year period, off‐channels of the lower Mokelumne River exhibited slower and warmer water than the main‐channel. Probability of salmonid presence varied by stream reach and habitat types. Steelhead and Chinook salmon both demonstrated transitional responses to the dry season, with juveniles leaving off‐channels by midsummer. This corresponded to flow recession, increasing water temperatures, salmonid growth and end of emigration period. Main‐channel steelhead observations continued until the following storm season, which brought cool flood flows to reconnect off‐channels and the next juvenile cohort of both species to the river. Within arid climates, low‐gradient off‐channels appear more transiently used than in cooler and more northern humid climate systems. Within a highly regulated Mediterranean‐climate river, off‐channel habitats become increasingly scarce, disconnected or temperature limiting in low‐gradient reaches both seasonally and due to anthropogenic modifications. These observations may provide guidance for future management within large salmon streams. Copyright © 2015 John Wiley & Sons, Ltd.     

Large dam removal and early spontaneous riparian vegetation recruitment on alluvium in a former reservoir: Lessons learned from the pre‐removal phase of the Sélune River project (France)

The restoration of ecological continuity along the Sélune River (Normandy, France) involves the removal of two tall hydroelectric dams (36 m removed in 2019 and 16 m in 2021), a project without precedent in Europe. During the pre‐removal phase (2014–2018), we performed scientific monitoring of the vegetation that was colonizing alluvium in the former dam reservoir (length: 19 km; surface area: 151 ha). Our study aimed to analyse if spontaneous vegetation could ecologically restore the riparian zone and help maintain fine sediment after dam removal. We used colonization indicators related to vegetation structure, taxonomic richness and diversity, and composition. These indicators were calculated at two spatial scales (local, at a single site, and broad, along the reservoir). The aim was to (a) characterize the spontaneously established species pool; (b) analyse longitudinal patterns in vegetation colonization; and (c) assess temporal changes in the species community. Our results show that diverse plant communities have developed. Slight differences in longitudinal and lateral patterns existed; they were linked with habitat heterogeneity and the reservoir's slow pace of draining. We observed fast spontaneous terrestrialization, which has resulted in cover stabilization, decreased diversity, and the development of herbaceous riverbank communities, with very few invasive species. This finding suggests stabilization potential is high and passive ecological restoration could occur, at least locally. Further analyses focusing on functional traits could help inform future management decisions regarding revegetation on reservoir alluvium.     

Process,Policy, and Implementation of Pool‐Wide Drawdowns on the Upper Mississippi River: A Promising Approach for Ecological Restoration of Large Impounded Rivers

The Upper Mississippi River (UMR) has been developed and subsequently managed for commercial navigation by the U.S. Army Corps of Engineers (USACE). The navigation pools created by a series of lock and dams initially provided a complex of aquatic habitats that supported a variety of fish and wildlife. However, biological productivity declined as the pools aged. The River Resources Forum, an advisory body to the St. Paul District of the USACE, established a multiagency Water Level Management Task Force (WLMTF) to evaluate the potential of water level management to improve ecological function and restore the distribution and abundance of fish and wildlife habitat. The WLMTF identified several water level management options and concluded that summer growing season drawdowns at the pool scale offered the greatest potential to provide habitat benefits over a large area. Here we summarize the process followed to plan and implement pool‐wide drawdowns on the UMR, including involvement of stakeholders in decision making, addressing requirements to modify reservoir operating plans, development and evaluation of drawdown alternatives, pool selection, establishment of a monitoring plan, interagency coordination, and a public information campaign. Three pool‐wide drawdowns were implemented within the St. Paul District and deemed successful in providing ecological benefits without adversely affecting commercial navigation and recreational use of the pools. Insights are provided based on more than 17 years of experience in planning and implementing drawdowns on the UMR. Copyright © 2015 John Wiley & Sons, Ltd.     

Patterns of species richness and introduced species in native freshwater fish faunas of a Mediterranean‐type basin: the Guadiana River (southwest Iberian Peninsula)

In this study, we analysed the factors affecting species richness and introduced species component patterns in native fish faunas of 30 streams of the Middle Basin of the Guadiana River. From a principal component analysis and a stepwise multiple regression analysis performed on a data matrix composed of ten hydrological and biotic variables, we showed that: (1) fish species richness increased with stream length and watershed area, (2) the number of native species in a stream declined as channelizations and river regulation (constructions of dams) are higher, whereas introduced species increased in the same way, (3) the two main negative factors affecting native ichthyofaunas affected dissimilar ecological areas: channelizations, which depend on land‐use intensity of floodplain, mainly occurred in lower reaches of streams, but construction of dams mainly took place in upper sections of rivers, (4) the length of the remaining well‐preserved reaches in a stream appeared to be the only factor accurately predicting native fish species richness, and (5) native fish faunas of small isolated streams are more vulnerable to habitat alteration than those of large streams. Both isolation and fragmentation of populations were recorded, so the conservation status of native and highly endemic fish fauna of the study area is extreme. Protection of the few still extant, well‐preserved small streams and upper reaches, habitat restoration of channeled areas, and inclusion of the need for native fish fauna conservation in long‐term public planning of water use become a priority. Fish communities appear to be a sensitive indicator of biological monitoring to assess environmental degradation. Copyright © 2001 John Wiley & Sons, Ltd.     

Hydrogeochemistry,Isotopic Composition and Water Age in the Hydrologic System of a Large Catchment within a Plain Humid Environment (Argentine Pampas): Quequén Grande River,Argentina

The Quequén Grande River (QGR) is a large catchment (10 000 km²) in the Pampa Plain in Argentina. From November 2004 to April 2013, a hydrochemical and stable isotopes monitoring program was conducted, which included three sampling stations of monthly composite precipitation, weekly samples in two sites along the river and several groundwater samples. A standard data interpretation was initially performed applying standard statistics, Piper diagrams and δ¹⁸O versus δ²H diagrams. The time evolution of the values of δ¹⁸O in precipitation and streamwater were also determined. The integration of hydrogeochemistry and stable isotopes data indicates the existence of three main components of streamflow: (i) baseflow characterized by electrical conductivity (EC) from 1200 to 1800 µs/cm and an isotope composition quite constant around δ¹⁸O ?5.3‰ and δ²H ?33.8‰. Water age for groundwater contribution is typically around 30 to 40 years using chlorofluorocarbons; (ii) direct runoff composed of channel interception and overland flow, which is of low EC in the order of 50 to 100 µs/cm, and a highly variable isotopic composition; and (iii) translatory flow (pre‐event water that is stored within the subsoil) with an intermediate EC and isotopic composition close to that of the weighted average composition of precipitation. The hydrochemical and stable isotopic data allow the differentiation between baseflow and direct runoff. In addition to this, chlorofluorocarbon dating is a useful tool in assessing the dominance of baseflow in a stream. The data lead to a conceptual model in which an intermediate flow system, with mean residence time (MRT) of around 35 years, discharges into the drainage network. A regional flow system (MRT > 50 years) discharges to the ocean. It is concluded that in this large plain catchment streamflow separation, only two components can be applied in: (i) short storm precipitation events having a high sampling frequency and (ii) during long dry periods when pre‐event soil water is not released. Copyright © 2016 John Wiley & Sons, Ltd.     

Water temperatures in the headwaters of the Volga River: Trend analyses,possible future changes,and implications for a pan‐European perspective

Water temperature is a physical property influencing abiotic and biotic parameters in an aquatic ecosystem. In different Central European rivers and lakes, a general trend of water temperatures in the range of +0.05 to +0.8 °C per decade was identified. Our case study analyses whether similar patterns apply to the headwaters of the Volga River, in the East European plain. Based on a dataset of water temperatures for 2008–2015, we investigated the spatial and temporal distribution of water temperature along the Tudovka River and estimated backward as well as predicted future changes from development scenarios throughout the 21st century. Stochastic models were applied to track trends and variations in water temperature. Furthermore, the correlation between water temperature and air temperatures was used to model historical water temperatures and to predict possible changes in the future, under the effects of climate change. Based on climate change scenarios, an increase of the mean water temperatures as well as changes regarding the ice cover can be expected until the end of this century. The conditions described for the headwaters of the Volga River system represent a valuable dataset for medium and large rivers in the East European plain and serve as a basis for future management.     

Variations in fish assemblages in a tributary of the upper Paraná River,Brazil: A comparison between pre and post‐closure phases of dams

This paper purposed to evaluate alterations in fish assemblage (structure and species richness) and variations in the abundance of the most abundant species captured in the Piquiri River, due to the closure of Porto Primavera (in 1998; located upstream the Piquiri River mouth) and the Itaipu (in 1982; located downstream) dams in the upper Paraná River. The effects of selected environmental variables were also determined. Fishes were sampled during four distinct periods. Six samplings were conducted in each period, at two different sites (Campina and Apertado). For sampling, gill and trammel nets were used. Species abundances were indexed by catch per unit of effort. Spatial and temporal changes in environmental variables were summarized by a principal components analysis (PCA). To summarize structure of the assemblage, a nonmetric multidimensional scaling (NMS) was used. To test the significance of between‐group differences at each periods and sites we used a multiresponse permutation procedure (MRPP). To test the association between fish assemblage and the environmental variables matrices, we used procrustean superimposition approach. A total of 2693 individuals were caught, belonging to 69 species, 4 orders and 19 families. Species richness differed spatially; the Campina site had greater mean richness (13) per sampling than the Apertado site (10). The structure of the fish assemblages showed significant temporal and spatial variations, which were more evident in the last period at the Apertado site (after the closure of Porto Primavera Dam). Spatial and temporal changes in structure of fish assemblages were not associated to environmental variables. Temporal changes in abundance, both in number and in weight, of the most abundant species were verified in both sites. The lack of migratory species was highlighted, which sharply decreased in the last studied period, when they were replaced by sedentary species. These findings appear to be related to regional factors, such as the decrease of floods registered in the upper Paraná basin, exacerbated by the control prompted by dams. Copyright © 2009 John Wiley & Sons, Ltd.