Regional, Synchronous Field Determination of Summertime Stream Temperatures in Western Washington

Abstract

Cold-water fisheries can be strongly affected by elevated summertime stream temperatures. The determinants of stream temperature are relatively well understood in the abstract, but their quantification in any given watershed is confounded by the vagaries of groundwater and surface-water inflows and the complex interplay of stream orientation and sun angle, canopy cover, and air temperature. Individual temperature measurements can give insight into the specific conditions for a particular stream, but they do not provide the context to evaluate unusual natural or human-induced temperature conditions at any given site.

To rectify this shortcoming, the Center for Urban Water Resources Management at the University of Washington, in cooperation with the Center for Streamside Studies and local stormwater agencies, tribes, and citizen groups, coordinated four intensive stream-temperature monitoring surveys, in mid-August 1998 and in early August of 1999, 2000, and 2001. Our intention was to characterize the range, distribution, and determinants of summertime high temperatures in fish-bearing (and tributary to fish-bearing) lowland stream systems in the Puget Sound lowlands. Sites were arrayed to provide system-wide coverage, with watershed areas ranging from over 200 km2 on down to the limits of perennial flow. Reflecting our interest in quantifying human influences, we targeted watersheds with primarily urban and suburban land uses but included some rural and forested basins as controls. About a dozen sites with continuous recording temperature gauges already installed were also covered to provide a temporal context for these "snapshot" data. Over 200 individuals, representing approximately 20 different agencies and community groups, collected temperature measurements across the south-central Puget Lowland in a two-hour period-over 500 sites in each year.