Determining the spatial variation of phosphorus in a lake system using remote sensing techniques

Lakes are versatile ecosystems, with eutrophication being a serious problem affecting their condition and trophic status. Eutrophication can lead to an over‐abundance of macrophytes in lakes, producing favourable conditions for mosquito larvae. Increased eutrophication is attributed in most to excessive phosphorus concentrations in lake water. Satellite imagery analysis now plays a prominent role for quickly assessing water quality over a large area. The present study is an attempt to illustrate the variation of phosphate and total phosphorus concentrations in Akkulam–Veli Lake, Kerala, India, using Indian Remote Sensing satellite (IRS P6‐ LISS III) imagery. A multiple regression equation derived using radiance in the red and MIR bands in the imagery was found to yield superior results for predicting the phosphate concentration, whereas a simple regression equation using radiance in red band was found to yield good results for the total phosphorus concentration in lake water. Accordingly, the trophic status of the lake system can be determined easily from satellite imagery in this manner.     

Deriving and Evaluating Bathymetry Maps and Stage Curves for Shallow Lakes Using Remote Sensing Data

Urmia Lake as a most vital water bodies in Iran, has been shrinking since the late twentieth century and its area has dramatically decreased. To develop and apply any plans to survive the lake, qualitative and quantitative analysis and any modeling, deriving physical information such as volume, area and their changes are very crucial. The objectives of this study were therefore, intended firstly, to study the bathymetry of Urmia Lake with a more satisfactory approach using Landsat- LDCM satellite image and in situ measurement data. The polynomial model was developed to predict the water depth in Urmia Lake area. This model was developed with the input series of reflectance values from blue, green, red and NIR bands in the Landsat- LDCM satellite imagery for Urmia Lake taken on 12 April 2013 of the sampling sites from actual depth measured were taken on the same date. Also, using a large archive of Landsat imagery (TM, ETM+ and LDCM), a counter of equivalent elevation were established for deriving the bathymetry of desiccated areas by mapping the edges of the lake and finally assembled with bathymetry derived from polynomial model. In-situ depth measurements were used to evaluate resultant derived bathymetric map. This comparison shows reasonable agreement between the Landsat-derived depths and those measured in the field with RMSE of 0.27 cm and R² = 0.91. The maximum and mean depths measured were 4.9 and 11 m respectively. The maximum depth measured was located at the upper part of the lake. As well as, developed multi-regression equation used for deriving another bathymetry map using Landsat- LDCM satellite image taken on Sep. 2015 for salt deposition monitoring. Results indicates that about 64 cm salt deposition has occurred during the last two years. Secondly, to make stage curves of lake, multi-temporal changes of water body have been derived from Landsat, MODIS and AVHRR satellite images sets since 1972. In this regard, the area of Urmia Lake at different level was estimated base on object oriented and pixel base classification using 78 satellite images. Finally, stage curve (volume- area- level relations) was derived from bathymetry map.     

Detection of Extent of Sea Level Rise in a Coastal Lake System using IRS Satellite Imagery

Coastal lakes are often under the threat of sea water ingress causing the depletion of fresh water bodies. Salinization due to sea water ingress causes impact on water resources especially in coastal areas. Thus there is scarcity of fresh water in this region. Climate change causes rise in sea level and this leads to increase of salinity in coastal lakes. Satellite imagery analysis now plays a prominent role in the quick assessment of water quality in a vast area. This study is an attempt to assess the extent of a lake system affected by sea water (Akkulam-Veli lake, Kerala, India) using Indian Remote Sensing Satellite, IRS P6 LISS III imagery. Field data were collected on the date of the satellite overpass. Simple regression equation using radiance in red band is found to yield superior results for the prediction of sodium content whereas polynomial equation using radiance of red band is found to yield superior results for the prediction of chloride content than the simple and multiple regression equations using spectral ratios and radiance from the individual bands from satellite imagery. The refractive index of water containing sodium chloride is more when compared with that of fresh water and this in turn results in high reflection of visible light in the red band when compared to blue and green bands. Hence the radiance of red band gives direct measure of sodium and chloride in lake water. IRS P6-LISS III imagery can be effectively used for the assessment of sea water affected area of the lake system.     

Computation of changes in the run-off regimen of the Lake Santa Ana watershed (Zacatecas, Mexico)

This study describes the application of a water run‐off model for Lake Santa Ana, Mexico, developed through the combined application of two simulation models: the Watershed Modelling System (WMS) and the Hydrologic Modelling System (HEC‐HMS). The WMS was used to estimate the geometric parameters of the hydrographic basin from a digital elevation model and geographical information describing the location of the hydraulic infrastructure developed in the basin in recent years. The HEC‐HMS was used to estimate the run‐off hydrographs, by using historical rainfall data and calibrating the model with the observed hydrometric data. The lake bathymetry was taken into account in order to estimate the lake's water balance. Recent observations indicate that anthropogenic activities have modified the natural run‐off features of the lake basin. The magnitude of these modifications was estimated by linking the WMS and HEC‐HMS models. Application of these linked tools allowed the successful estimation of the modified basin limits and hydraulic behaviour of Lake Santa Ana.     

Three methods for determining the area–depth relationship of Lake Poopó, a large shallow lake in Bolivia

Lake Poopó in Bolivia is a large, extremely shallow lake. The lake surface area is ≈3000 km² at its spill‐over level, although it could dry out at times. The lake water level varies with the regional rains and with the water inflow from the Desaguadero River. This study focuses on using and comparing three different approaches to determine the hypsographic curve for the lake. As the lake is extremely shallow, depth measurements must be conducted from a slowly moving boat using a rod. The accuracy using echo‐sounding of the lake is poor. The second approach, which can be applied because the lake water level can vary considerably, and because the lake sometimes is almost dry, is to relate the lake surface area determined from satellite images to elevations determined from benchmarks on an island in the lake. The third method is an indirect one, in which the change in lake water volume over the period between two satellite images is determined from water balance computations and subsequently related to changes in the water surface area, as determined from the satellite images. Determination of the shoreline for different water levels in the lake is affected by errors related to the tilting of the water surface, and because wind forces the water to move several hundred metres towards the windward shore of the lake during a measurement survey.     

Investigation of physical and bathymetric characteristics of Lakes Abaya and Chamo, Ethiopia, and their management implications

The purpose of this study was to investigate the physical parameters of Lakes Abaya and Chamo in the Ethiopian rift lakes system, including such physical characteristics as depth, water resources capacity, hydrology, water balances, and impacts of water use and degradation of their watersheds. These parameters have not previously been studied for these two lakes to any significant extent. This study describes the bathymetry survey undertaken for these two lakes, and the morphometric characteristics derived from it. This study is part of a research project developed to provide further details on such parameters as hydrology, water quality, sediment inflows and deposition, lake hydrodynamics and consumptive water uses. The bathymetric survey was conducted, utilizing a combination of global positioning system (GPS) and echo sounder. To calculate the morphometric characteristics, the background lake map was digitized, and the surveyed primary data were developed as digital values. The digital values were interpolated, generating grids of the elevation surface. The elevation area and elevation volume curves (capacity curves) of the two lakes were developed from the digital values, describing the water resources capacity of the lake water basins. The results of this study increase our understanding of the water resources of these two lakes, as well as provide better understanding of their vulnerability to human activities because of their shallow depths. Immediate application of the results, as a basis for continuation of this study, also is highlighted.     

Quantification of channel bed morphology in gravel-bed rivers using airborne multispectral imagery and aerial photography

The potential for mapping in-channel morphology within shallow gravel-bed rivers using airborne multispectral imagery and aerial photography is illustrated using a case study from the River Tummel, Scotland. The technique described relies on a good correlation between observed light reflectance levels from a water body and water depth. Measured water depths are regressed against reflectance levels derived from airborne multispectral imagery and black-and-white aerial photographs, to obtain equations that can be used for mapping channel bathymetry. The technique has a great deal of potential for wide-ranging applications, including detailed morphological surveys, assessing in-channel changes and mapping riverine habitats.© 1997 John Wiley & Sons, Ltd.     

Mapping and monitoring the extent of submerged aquatic vegetation in the Laurentian Great Lakes with multi-scale satellite remote sensing

A satellite-based algorithm intended to map submerged aquatic vegetation (SAV), which was mostly the nuisance algae Cladophora, for the Laurentian Great Lakes has been developed and successfully demonstrated in test areas in Lakes Michigan and Ontario. The new Submerged Aquatic Vegetation Mapping Algorithm (SAVMA) first utilizes deep water (opaque) radiance values to correct shallow water values (transparent) so that depth invariant reflectance values for all three visible Landsat bands of the lake bottom can be classified. Combinations of two bands are then used to generate a depth invariant bottom type index. The algorithm then maps the lake bottom into three types: sand, dense SAV, less dense SAV by thresh-holding the depth invariant reflectance values. The SAVMA also generates a biomass estimate by assigning an average dry weight obtained by field sampling to both the dense and less dense SAV areas identified by the algorithm.The algorithm performance was successfully evaluated on Lake Michigan at the Sleeping Bear Dunes National Lakeshore (SBDNL) using Cladophora locations provided by diver survey as well as from an independent National Park Service monitoring. The SAVMA correctly mapped Cladophora to an approximate accuracy of 85%, where the misclassification was a result of mixed pixels due to the resolution of the Landsat data and imprecise atmospheric corrections. The algorithm was also successfully evaluated in Lake Ontario near Pickering, ON. The SAVMA was then used to generate both short and long term time-series analyses of Cladophora extent at SBDNL.     

Impacts of wind waves on sediment transport in a large,shallow lake

Lake Okeechobee is a large, shallow subtropical lake, located in south Florida. Over the last several decades, Lake Okeechobee has experienced accelerated eutrophication due to excessive phosphorus loads from agricultural run‐off. Recycling of phosphorus from bottom sediments through resuspension is critical to addressing eutrophication of the lake and for water quality management. The present study investigates the impacts of wind waves on sediment transport in Lake Okeechobee, using measured data and the Lake Okeechobee Environmental Model (LOEM). The LOEM was fully calibrated and verified with more than 10 years of measured data in previous studies. Analysis of the measured data indicates significant wave height (SWH) and suspended sediment concentration are closely correlated to the wind speed in the lake. The nonlinear interaction of high‐frequency wind waves with relatively low‐frequency currents in the boundary layer plays a key role in sediment deposition/resuspension. Without considering the effects of wind waves, the bottom shear stress can be greatly underestimated. The spatial variations of key variables for sediment modelling, including SWH, water depth, orbital velocity, current velocity, bottom shear stress and sediment concentration, are discussed. In general, the near‐bottom wave velocity (and the associated bottom shear stress) is greater than or the same order of magnitude as the near‐bed current velocity (and the associated bottom shear stress) in this shallow water system. Although the sediment zones of Lake Okeechobee were described in previous studies, few published papers discussed its formation mechanisms. The findings of the present study include that the multiyear averaged bottom shear stress with wind‐wave effect plays a key role in forming the spatial patterns of the sediment zones. The study results are currently being used in lake management and in developing strategies for reducing phosphorus in the lake.     

Comparison of empirical and theoretical remote sensing based bathymetry models in river environments

Knowledge of underwater morphology is an essential component of many hydrological and environmental applications such as flood modelling and lotic habitat mapping. Remote sensing allows modelling of bathymetry at spatial scales that are impossible to achieve with traditional methods. However, the use of passive remote sensing for modelling water depth in fluvial environments remains a challenge. Different methods of computing bathymetry models based on remotely sensed imagery combined with ground measurements for calibration were investigated in order to produce a digital bathymetry model of a reach of the river Tana in Lapland. An empirical deep water correction model was evaluated together with theoretical hydraulically assisted bathymetry (HAB) models. The empirical model produced good results, correlating to known depths at 0.98 (R ² = 0.96) with a mean error of ±12.0 cm. It was demonstrated that usable levels of accuracy can be achieved with data that had previously been considered unsuitable for bathymetry modelling. Some issues related to channel substrate were addressed. The models based on hydraulic theory were tested for the first time outside the area they were developed in. Both models were found to be rather sensitive to certain assumptions, such as the channel friction parameter. The HAB models are able to produce relative depth estimates that can under certain conditions approach actual depths at accuracies similar to the empirical model. Extensive accuracy assessment was performed in order to evaluate the vertical as well as the spatial accuracy of the three models. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling natural conditions and impacts of consumptive water use and sedimentation of Lake Abaya and Lake Chamo, Ethiopia

There is few available information regarding the water resource systems of Abaya Lake and Chamo Lake, which are found in the Southern Rift Valley Region of Ethiopia. This paper describes modelling of the water balance components of these lakes, as well as the impacts of water uses, and sediment transport and deposition in the lakes. The various parameters and data needed for the water balance model are derived on the basis of various surveys, analysis of data and modelling efforts. The watershed characteristics are derived using geographical information system, whereas the morphometry of the lakes is investigated by undertaking bathymetry surveys. The hydrometeorological components of this lake system also were investigated through the development of relevant database and information systems, by identifying regional relationships, and by a rainfall‐run‐off model. These information systems have subsequently been integrated to model the water balance of the two lakes, and simulating the in‐lake water levels. Several scenarios reflecting the natural conditions, water consumptive development possibilities, and sedimentation impacts have been investigated in this study. Based on the model simulation results, and on the computation of the life expectancies of the two lakes, it was found that sediment inflow and deposition significantly threaten their existence.     

EVALUATING SHALLOW‐WATER BATHYMETRY FROM THROUGH‐WATER TERRESTRIAL LASER SCANNING UNDER A RANGE OF HYDRAULIC AND PHYSICAL WATER QUALITY CONDITIONS

The fine‐scale structure of the water–sediment boundary in fluvial environments is dynamic and complex, influencing near‐bed flows, sediment transport and instream ecology. However, accurate high‐resolution surveying of marginally or partially inundated areas of river channels is problematic. Previous work has shown that terrestrial laser scanning (TLS) through relatively shallow‐water columns using standard green‐wavelength equipment introduces errors of <5 mm in a static, clear water column. This paper presents seven laboratory and field tests of through‐water TLS under variable flow velocities, depths, suspended sediment concentrations, water colour levels and scan ranges. Flow velocity decreased point accuracy only for supercritical flows, whereas point density decreased as a function of both water depth and suspended sediment concentration. A similar point return threshold was observed for water colour variations with no grains in suspension. Conversely, point precision and accuracy were a function of suspended sediment concentration alone (a threshold of 0.11 g L^?1 was observed). Field tests showed larger errors (<10 mm) and lower point precisions. A clear‐water depth‐penetration limit of 0.68 m was identified. Fluvial bathymetry acquired from through‐water TLS is presented for a gravel/boulder bed reach. Despite observed limits, these experiments demonstrate that our approach provides centimetre‐resolution bathymetry and sub‐aerial survey in an integrated dataset without the need for the following: (i) additional financial resources; (ii) concurrent depth measurements; or (iii) extra field effort for bathymetry acquisition, thereby enabling regular surveys to characterize the fine‐scale structure of channel beds and to constrain the geomorphic effect of individual flood events. Copyright © 2013 John Wiley & Sons, Ltd.     

Hyperspatial Remote Sensing of Channel Reach Morphology and Hydraulic Fish Habitat Using an Unmanned Aerial Vehicle (UAV): A First Assessment in the Context of River Research and Management

In this paper, we assess the capabilities of an unmanned/uninhabited aerial vehicle (UAV) to characterize the channel morphology and hydraulic habitat of a 1‐km reach of the Elbow River, Alberta, Canada, with the goal of identifying the advantages and challenges of this technology for river research and management. Using a small quadcopter UAV to acquire overlapping images and softcopy photogrammetry, we constructed a 5‐cm resolution orthomosaic image and digital elevation model (DEM). The orthomosaic was used to map the distribution of geomorphic and aquatic habitat features, including bathymetry, grain sizes, undercut banks, forested channel margins, and large wood. The DEM was used to initialize and run River2D, a two‐dimensional hydrodynamic model, and resulting depth and velocity distributions were combined with the mapped physical habitat features to produce refined estimates of available habitat in terms of weighted usable area. Based on 297 checkpoints, the vertical root‐mean‐squared error of the DEM was 8.8 cm in exposed areas and 11.9 cm in submerged areas following correction of the DEM for overprediction of elevations as a result of the refractive effects of water. Overall, we find several advantages of UAV‐based imagery including low cost, high efficiency, operational flexibility, high vertical accuracy, and centimetre‐scale resolution. We also identify some challenges, including vegetation obstructions of the ground surface, turbidity, which can limit bathymetry extraction, and an immature regulatory landscape, which may slow the adoption of this technology for operational measurements. However, by enabling dynamic linkages between geomorphic processes and aquatic habitat to be established, we believe that the advantages of UAVs make them ideally suited to river research and management. Copyright © 2014 John Wiley & Sons, Ltd.     

Integrating economic and hydrologic interdependence in reservoir management

The purpose of this study is to determine the optimal allocation of reservoir water among competing uses (hydroelectric power generation, lake recreation and urban and rural water supply). An optimization model using nonlinear programming is developed to optimally allocate reservoir water among these uses. It is not unusual to optimize the values for flood control, hydroelectric generation, and urban and rural water uses, and to determine recreation values as a residual. Furthermore, if recreation values are considered, it may be in the form of constraints that maintain reservoir water levels within a specified range. In contrast, this study develops a model in which the recreational benefits depend explicitly on lake water levels, while the flood control capacity of the reservoir is maintained through upper bounds on the lake level. A mass balance equation is used to determine the water levels and volumes in the lake for each month over a twelve‐month period. The General Algebraic Modeling System with MINOS is used to solve this model. The results indicate that the total economic benefits for Lake Tenkiller could be increased by directly including recreational values in the model, and maintaining lake water levels at near‐normal levels that maximize the number of summer visitations. The optimal allocation of Lake Tenkiller water among competing uses also satisfies the equimarginal principle. That is, the marginal value of water at the lake in each month is the same for the last unit of water used for hydroelectric, recreational, or urban and rural water uses. Use of this type of modelling framework would assist policymakers or reservoir managers in reallocation of reservoir water and in calibrating several policy scenarios.     

How light conditions influence theoretical pelagic to benthic primary production ratios in small lakes

Theoretical pelagic primary production of phytoplankton and benthic primary production of periphyton were modelled for two small lakes in Estonia (Northeast Europe). Although located only 500 m apart, the water colour and light attenuation of these two lakes differed markedly. The Secchi depth (SD) in the clear‐water lake was 4.5 m and only 0.47 m in the dark‐water lake. The total phosphorus (TP) concentrations were, respectively, 15 μg/l and 28 μg/l. An empirical model whose inputs were morphometric, light conditions and dissolved organic carbon parameters obtained from in situ measurements was employed for the present study. The model calculated primary production with a time‐step of 10 min, and a spatial resolution of 10 cm, from sunrise to sunset and from lake surface to lake bottom. The primary production of periphyton and phytoplankton was almost equal in the clear lake, whereas only phytoplankton contributed to whole‐lake primary production in the dark lake because of the stronger light attenuation in the water column. The results of the present study indicated the depth‐distribution profiles differed dramatically between the two lakes. The clear lake had a deep, U‐shaped curve, with the productive layer reaching considerable depth soon after sunrise and maintaining a similar profile throughout the light hours. In contrast, the dark lake production declined rapidly with increasing depth, whereas the profile changed over the day reaching the greatest depth at noon.     

Satellite based lake bed elevation model of Lake Urmia using time series of Landsat imagery

This study presents a lake bed elevation model of Lake Urmia. In the course of model generation, a time series of the extent of the lake surface was derived from 129 satellite images with different acquisition dates based on the Landsat sensors Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), and Operational Land Imager (OLI). Due to the rapid shrinking of the lake during the last two decades, lake surface areas ranging from 890 km² to 6125 km² could be covered. The water edge of the various lake extents was then linked to the observed water level on the day of the satellite image acquisition. The resulting contour lines, covering water levels between 1270.04 m and 1278.42 m a.s.l. and thus representing the lakebed morphology in its shallow parts, were merged with existing data (deeper parts) and interpolated to generate a lake bed elevation model. Finally, Lake Urmia’s Level-Area-Volume relationships were derived from the lake bed elevation model and compared to bathymetric data previously published.     

The changing face of the Ethiopian rift lakes and their environs: call of the time

The Ethiopian rift is characterized by many perennial rivers, and a chain of lakes that vary in size, and hydrological and hydrogeological settings. The water resources of the Rift lakes are one of the focal points for large‐scale development in Ethiopia over the last few decades. Some of the lakes and their influent rivers are used for irrigation, soda abstraction, fish farming and recreation, and also support a wide variety of endemic birds and wild animals. Ethiopia's major mechanized irrigation farms and commercial fishery are confined within the Rift region. A few of the lakes have shrunk in surface area because of excessive water abstractions, while others have expanded because of increased surface run‐off and groundwater inflows from percolated irrigation water. Excessive land degradation, deforestation and over‐irrigation have changed the hydrological setting of a few Rift lakes. Human activities, in combination with changes in climate and geology, have influenced the hydrological setting and the water quality of the lakes, with the salinity and major ion composition dramatically changed in some of them. This study tries to address the challenges associated with development of these surface‐water resources, focusing on environmental problems arising over the past few decades on three lakes (Abiyata, Beseka and Ziway) situated along the tectonically active Rift floor. The methods utilized in this study include field hydrological and hydrogeological mapping, supported by aerial photographs and satellite imagery interpretations, as well as hydrometeorological and hydrochemical data analysis, and catchment hydrological modeling. A converging‐evidence approach was adapted to reconstruct the temporal and spatial variations of the lake water levels and surface areas. The study results revealed that the major changes in the Rift Valley are related primarily to recent improper use of water from the large rivers draining the Rift Valley floor and the lake catchments, and from direct lake water abstractions aggravated intermittently by climatic and land use changes. These changes represent grave environmental consequences on the fragile Rift ecosystem that demands urgent intervention in the context of an integrated, basin‐wide water management approach. This study emphasizes lake water level changes and human influences on these changes. It also assesses human interactions and water quality changes, including land use changes and environmental repercussions on the lakes, as well as providing recommendations on how these issues should be addressed.     

Stratification regimes and thermodynamic modelling of a subtropical African reservoir

The stratification of a lentic water system is an important factor regulating its biotic activities and processes. The present study investigated changes in temperature and oxygen stratification regimes in Mazvikadei Reservoir 27 years after the first study was conducted during its filling phase. The FLake‐Global model was also tested as a predictive tool for the first time to analyse annual limnological trends in a subtropical reservoir. The lake‐modelling tool FLake‐Global enables an instantaneous estimation of the seasonal cycle of temperature and mixing conditions in shallow freshwater lakes around the world. The results of the present study illustrate that the reservoir was weakly stratified, with the overall thermal stratification patterns somewhat similar to those recorded during the filling phase (1992). The oxygen stratification patterns observed in the present study correspond well to those of thermal stratification. The depth‐integrated oxygen profiles for both the deep and shallow sampling sites exhibited the same pattern, with higher concentrations from July to September. The water column was well oxygenated, indicating the reservoir has matured since its filling phase in 1992, when there was significant hypolimnetic deoxygenation. The FLake‐Global model exhibited good results in predicting annual trends, with the results also indicating no differences in stratification levels in the reservoir for the current and Masundire (1992) filling phase studies. The model predicted lower surface and bottom temperatures, however, compared to those measured in the reservoir. There was lowering of the mixed layer depth from the hot‐wet (November–April) months through the cool‐dry (May–August) season. Latent heat fluxes agreed with the net longwave (LW) predictions for Mazvikadei Reservoir, which exhibited an overall net cooling effect. The stratification has not changed significantly since the previous study in 1988. The FLake‐Global model proved a useful predictive tool when tested in a subtropical system.     

Spectrally based bathymetric mapping of a dynamic,sand‐bedded channel: Niobrara River,Nebraska, USA

Methods for spectrally based mapping of river bathymetry have been developed and tested in clear‐flowing, gravel‐bed channels, with limited application to turbid, sand‐bed rivers. This study used hyperspectral images and field surveys from the dynamic, sandy Niobrara River to evaluate three depth retrieval methods. The first regression‐based approach, optimal band ratio analysis (OBRA), paired in situ depth measurements with image pixel values to estimate depth. The second approach used ground‐based field spectra to calibrate an OBRA relationship. The third technique, image‐to‐depth quantile transformation (IDQT), estimated depth by linking the cumulative distribution function (CDF) of depth to the CDF of an image‐derived variable. OBRA yielded the lowest depth retrieval mean error (0.005 m) and highest observed versus predicted R² (0.817). Although misalignment between field and image data did not compromise the performance of OBRA in this study, poor georeferencing could limit regression‐based approaches such as OBRA in dynamic, sand‐bedded rivers. Field spectroscopy‐based depth maps exhibited a mean error with a slight shallow bias (0.068 m) but provided reliable estimates for most of the study reach. IDQT had a strong deep bias but provided informative relative depth maps. Overprediction of depth by IDQT highlights the need for an unbiased sampling strategy to define the depth CDF. Although each of the techniques we tested demonstrated potential to provide accurate depth estimates in sand‐bed rivers, each method also was subject to certain constraints and limitations.