Development and Evaluation of a Habitat Rating System for Low-Gradient Wisconsin Streams

Abstract

Most existing habitat indices and models have been developed for medium- to high-gradient streams and, therefore, tend to undervalue low-gradient streams. We analyzed physical habitat and fish community biotic integrity data from 50 low-gradient stream stations to develop a habitat rating index for the assessment of low-gradient streams in Wisconsin. Low-gradient stations were defined as having gradients of 3 m/km or lower and less than 5% of their length as riffles. Of 43 habitat variables considered, we selected 7 for use in the index: percent channelization, channelization age, percent instream cover, percent bank erosion, sinuosity, standard deviation of thalweg depth, and percent buffer vegetation. Based on a multiple-regression analysis, channelization age and percent instream cover were given greater weight in the index than the other five variables. Habitat index values explained 58% of the variation in index of biotic integrity (IBI) scores among the 50 stations used to develop the index and explained 64% of the variation in IBI scores for an independent test data set of 42 other low-gradient stations. In contrast, a habitat index designed for medium- to high-gradient streams explained only 47% of the variation in IBI scores for these 42 other low-gradient stations. We conclude that our new index more accurately represents the quality of physical habitat for fish communities in Wisconsin low-gradient streams and suggest that it also may be useful for assessing low-gradient streams elsewhere in North America.