Effects of Winter Floods on Fishes in the Sierra Nevada

Abstract

Winter floods in the Sierra Nevada mountains kill age 0 class brook trout (Salvelinus fontinalis) and Paiute sculpin (Cottus beldingi) because bed-material transport increases greatly when high flows are constrained by snow banks. In February 1982, dead Paiute sculpin were collected while sampling bedload during a rain-on-snow flood. Population estimates by electrofishing at nine permanent stations the following summer showed that density (3586∙ha−1) and biomass(12.9 kg∙ha−1) of Paiute sculpin were lower than the respective means (12 017∙ha−1 and 40.3 kg∙ha−1) obtained during previous studies from 1952 to 1961. These estimates were also below those obtained in 1956, after the largest winter flood from 1952 to 1961. Brook trout fry were also less abundant in 1982 than previously reported. Maximum flow depth, rather than discharge, were the likely cause offish mortality. Winter floods are severe because accumulated snowpack increases the effective height of the streambank and confines most or all of a rain-on-snow flood within the channel. Shear forces on the bed increase and as a result bed-material transport increases rapidly. These conditions directfy kill many benthic-living fishes such as the Paiute sculpin or buried eggs of fall-spawning fishes such as the brook trout by mechanical grinding or crushing. The impact of snow-constrained floods was not uniform along Sagehen Creek due to patchiness in types of riparian canopy. The relationship among canopy cover, snow accumulation, and winter floods points to one more critical role that buffer strips may play in ameliorating effects of timber harvesting.