Thermal Profiling of Long River Reaches to Characterize Ground-Water Discharge and Preferred Salmonid Habitat

The thermal regime of riverine systems strongly influences the species composition, trophic structure, and population dynamics of aquatic ecosystems. Ground-water discharge is an important component of the thermal regime, providing preferred thermal structure for fish with different life histories, such as salmonids, and refugia during seasonal extremes of water temperature. In large, diverse river basins, documenting the thermal regime and locating the areas of ground-water discharge has been difficult. A method was developed to profile the thermal structure of long (on the order of 8 to 25 kilometers) river reaches by towing thermistors from a boat that sample near-surface and near-bed temperatures at 1- to 3-second intervals, while a Geographic Positioning System records spatial coordinates. This method was developed, tested, and applied to the Yakima River Basin, Washington. Seven reaches were profiled in the summer of 2001 during low flows of an extreme drought year. A total of 146 kilometers of river was profiled over 7 days. The thermal profile provided valuable information on the spatial and temporal variation in habitat and, notably, indicated areas of ground-water discharge. The areas of ground-water discharge were typified by a temperature decrease in summer and an increase in winter. The spatial distribution of the river's temperature structure determined using this method cannot be captured by fixed-station or synoptic data. The profiles exhibit inter- and intra-reach diversity that reflects the many factors controlling the temperature of water as it moves downstream. These profiles provide a new perspective on the temperature regime of a riverine system that represents part of the aquatic habitat template for lotic community patterns, including a logical progression of the longitudinal gradient of fish assemblages.