A hypothesis about factors that affect maximum summer stream temperatures across montane landscapes

Temperature is an important variable structuring lotic biotas, but little i s known about how montane landscapes function to determine stream temperatu res. We developed an a priori hypothesis that was used to predict how water shed elements would interact to affect stream temperatures. The hypothesis was tested in a series of path analyses using temperature data from 26 site s on second-order to fourth-order streams across a fifth-order Rocky Mounta in watershed. Based on the performance of the first hypothesis, two revised versions of the hypothesis were developed and tested that proved to be mor e accurate than the original hypothesis. The most plausible of the revised hypotheses accounted for 82 percent of the variation in maximum stream temp erature, had a predicted data structure that did not deviate from the empir ical data structure, and was the most parsimonious. The final working hypot hesis suggested that stream temperature maxima were directly controlled by a large negative effect from mean basin elevation (direct effect = -0.57, p < 0.01) and smaller effects from riparian tree abundance (direct effect = -0.28, p = 0.03), and cattle density (direct effect = 0.24, p = 0.05). Wate rshed slope, valley constraint, and the abundance of grass across a watersh ed also affected temperature maxima, but these effects were indirect and me diated through cattle density and riparian trees. Three variables included in the a priori hypothesis - watershed aspect, stream width, and watershed size - had negligible effects on maximum stream temperatures and were omitt ed from the final working hypothesis.