![]() ![]() Rivers with dense submerged aquatic vegetation (SAV) may also experience thermal stratification, as SAV steepens vertical temperature gradients and decreases water velocity. In contrast, in unconfined sections, where changes in wetted perimeter and channel width are largest, warmer water temperatures are the result of a larger surface area exposed to solar radiation. At high flow velocities, water may be cooler due to mixing and topographic shading, but at low velocities, thermal stratification may occur in the summer when surface water temperature is higher than deeper water. In confined sections of a river where the river is narrow and deep, two main mechanisms may be at play, depending on flow velocity. The mechanisms by which channel geometry (i.e., water depth, wetted perimeter, stream width and cross-sectional area) indirectly affect water temperature are complex. Changes in water depth and flow velocity are likely greatest in confined valleys, whereas changes in the extent of wetted perimeter and channel width may be largest in unconfined valleys, such as in the Colorado, Green and Missouri rivers. However, dam effects on flow and sediment are also dependent on the geomorphic setting where lateral and vertical movement of flow and sediment are constrained by valley shape. Water releases from the dam that have a limited sediment load and excess energy may incise the channel bed and coarsen the channel bed material. In impounded rivers, flow velocity, water depth, and the extent of channel wetted perimeter are decoupled (spatially and temporally) from the river’s geomorphic setting due to dam operations. These indirect modifications to the thermal regime occur when dams alter sediment transport and channel geometry and fragment riparian and riverine habitat. Indirect modifications of the thermal regime are those that affect the processes of heat exchange between water and the environment. When taking into consideration the geomorphic setting, confined valleys are likely to be more susceptible to changes in water temperature due to water releases than are unconfined valleys, which have greater surface water-groundwater exchange and greater hydrologic storage. Direct modifications to the thermal regime are made by upstream water releases and dam operations. The thermal effects of dams have been studied for many years, and it is recognized that these effects depend on the landscape position of the dam, dam operation, water release depth, and geomorphic setting. Large dams can be intentionally managed to release cold water from deep reservoirs to provide habitat for salmonid fisheries, whereas releases from any size dam and diversion can increase downstream temperatures by releasing warm water directly from the reservoir surface. Specifically, dams and diversions can greatly modify riverine thermal regimes by restricting or eliminating connectivity in longitudinal, lateral, vertical, and temporal dimensions. In rivers and streams where humans have altered the thermal regime, not only does summer water temperature frequently exceed the thermal tolerance of native salmonids, thus influencing their distribution, but these human modifications can amplify or dampen spatial and temporal variation. These findings highlight the importance of using multiple approaches to describe thermal heterogeneity in large impounded rivers and the need to incorporate these types of rivers in the understanding of thermal riverscapes because of their limited representation in the literature. We demonstrated that (1) lateral contributions from tributaries dominated thermal heterogeneity (2) thermal variability at confluences was approximately an order of magnitude greater than of the main stem (3) potential cold-water refuges were mostly found at confluences and (4) the probability of occurrence of cool areas and median water temperature were associated with channel geomorphology and distance from dam. We explained the median water temperature and probability of occurrence of cool-water areas using generalized additive models (GAMs) at reach and sub-catchment scales, and we evaluated potential cold-water refuge occurrence in relation to these patterns. To describe these patterns, we used thermal infrared (TIR) imagery, in situ thermographs, and high-resolution 3-D hydraulic mapping. ![]() We examined thermal heterogeneity across space and time and identified potential cold-water refuges for salmonids in a large impounded river in inland northwestern USA. This occurs by restricting or eliminating connectivity in longitudinal, lateral, vertical and temporal dimensions. Dam operations can affect mixing of the water column thereby influencing thermal heterogeneity spatially and temporally. ![]()
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