Dj. Isaak et Wa. Hubert, A hypothesis about factors that affect maximum summer stream temperatures across montane landscapes, J AM WAT RE, 37(2), 2001, pp. 351-366
Citations number
57
Categorie Soggetti
Environment/Ecology
Journal title
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
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.