Relationship of Riparian Buffer Type to Water Temperature in the Driftless Area Ecoregion of Minnesota

Abstract

We used the U.S. Fish and Wildlife Service's Stream Network Temperature Model to examine the role of riparian buffer type in mediating summer water temperatures for the reintroduction of brook trout Salvelinus fontinalis into Wells Creek, a tributary to the Mississippi River located in southeastern Minnesota. Stream temperatures measured from 23 July to 3 September 1997 were used to calibrate the model, evaluate existing temperatures, generate simulations for different shade conditions and channel morphologies, and generate simulations for “average” and “warm” summers (we define a warm summer as one that is 2.8°C above the 30-year mean). The simulations indicated that successional buffers (grasses and forbs) provided as much shade as wooded buffers in streams with a width less than 2.5 m. With a low width:depth ratio, the successional buffer vegetation mediated mean temperature as well as the wooded buffer when discharge was held constant. At a discharge characteristic of our study reach, the mean temperature would be about the same along a successional buffer as in a wooded buffer if the wooded vegetation also led to widening of the stream channel. However, wooded buffers had a significantly higher percentage of shade than grazed or successional buffers. In general, temperatures in an average year decreased along the wooded reaches and increased slightly along the successional and grazed buffer areas. The differences in measured weekly mean maximum temperatures may be as great as 2.5°C across riparian buffer types. Maximum temperatures across all riparian types would be higher during warmer than average years, even with 50% shade along the stream. Shade provided by successional and woody vegetation may serve to moderate maximum temperatures and may be sufficient for the reintroduction of brook trout if other conditions improve.