WATERSHED ANALYSES AND 15N TRACER EXPERIMENTS LANDSCAPE PATTERNS OF STREAMS & LAKES IN MONTANE WATERSHEDS DETERMINE WATER TEMPERATURES & NUTRIENT TRANSPORT: WATERSHED ANALYSES AND 15N TRACER EXPERIMENTS Wurtsbaugh, Wayne1., J. Garrett, G. Burkart1, W. Fleenor2, K. Nydick1, R. Hall3 and M. Baker1 Utah State University (wurts@cc.usu.edu); 2U. of California Davis; 3U. of Wyoming, USA Mountain watersheds differ in the number and distribution of lakes. In the Sawtooth Mountains of Idaho (USA), mean interlake distance is 2.8 km, but some watershed have just a few cirque lakes (Stanley), others have large lakes throughout the drainage (Yellow Belly Creek), whereas others have almost no lakes (Beaver Creek) (Figure 1) Abstract  In glaciated mountains, lakes are interspersed through watersheds and connected by streams. Although lakes or streams are frequently studied as individual water bodies, studying them as integrated functional units provides considerable insight on temperature patterns, nutrient transport and other functions. In the Sawtooth Mountains (Idaho), inter-lake distance averages 2.8 km. In summer, lakes are solar collectors, and warm outflow streams as much as 10 C, thus increasing metabolic rates. These streams seldom cool to equilibrium temperatures before encountering another lake, where the waters tend to overflow and mix into the epilimnion. With overflow, N-15 tracer experiments demonstrated that water and nutrients pass quickly through the epilimnion to the outflow stream where they can be taken up by periphyton. In contrast, in watersheds without upstream lakes, cold stream inflows plunge into the metalimnia and bring nutrients to the deep chlorophyll layer where they are retained, thus reducing downstream transport to other lakes and stream reaches as much as 5-fold. A landscape approach is consequently necessary to understand how the lakes and streams in these watersheds function. N Salmon River Yellow Belly Stanley Creek Watershed 2 km Beaver Creek Laks are big solar collectors! % of surface water area in: Stream Lake Beaver Cr. 75% 25% Stanley Cr. 18% 82% Yellow Belly 6% 94% Thermistors placed along stream courses show how lakes heat the water, whereas streams are cooled by entering groundwater. Nevertheless, lake heating has sustained downstream effects. 15NH3 added to inflowing stream water demonstrated that the cold plunging inflows deliver nutrients to the metalimnia of lakes, thus supporting algae in the deep chlorophyll layers Seston 15N profiles Mean August Temperatures Rhodamine tracer experiments show that water warmed by upstream lakes is bouyant and overflows into the epilimnia of lakes lower in the drainage. Streams with few lakes in the drainage are cold and dense, and plunge into the metalimnia of lower lakes.