CONSEQUENCES OF INCREASED LOW AND MEAN STREAMFLOW IN MIDWESTERN AGRICULTURAL WATERSHEDS ON GEOMORPHIC - ECOLOGICAL INTERACTIONS

There have been significant increases in streamflow over much of the upper Midwest in recent decades. Traditionally, increased flows below the 50^th percentile have been considered to have a positive impact on in-stream ecological processes. However lack of summer drawdown may negatively impact the life cycle of many aquatic organisms. For example, riparian tree colonization on sandbars and nesting of river turtles are dependent on low flows occurring in early to mid-summer.

We used field methods and hydrologic analysis on five rivers within eastern Minnesota (the Minnesota, Kettle, St. Louis, Cannon, and Root Rivers) including surveys of turtle nests, sandbar particle size, slope and temperature measurements, Indicators of Hydrologic Alteration (IHA) regime change assessment, and estimation of the frequency of sandbar emergence. Ongoing work includes assessment of change in riparian plant species establishment based on a comparison to a 1970s study done on riparian tree establishment in the Minnesota River.

The hypothesis that increased low to mean flows have decreased the duration and frequency of turtle nesting opportunity was tested on the above rivers. Hydrologic analysis showed there has been substantial hydrologic alteration in southern Minnesota but not in the northern forested region. The increased magnitude and duration of summer flows has decreased the frequency and duration of sandbar exposure. There has been a shift not only in magnitude but in the regime itself as indicated by the ratio of summer mean flow to annual mean change as and other indicators. Impacts on the turtle life cycle include increased stress, decreased time available for nesting and likely decreased turtle recruitment until the channel reaches a new equilibrium.

In conclusion, increased low and mean flows can have ecological impacts as great as changes to flood flows due to disruptions in aquatic organisms’ life cycles. Future research is needed on the effect of increased mean flows on sediment transport and channel evolution. Channel evolution models need to account for more than just physical processes of bank collapse, but changes in the seasonality, duration and frequency of stream hydrographs, which may effect vegetative interactions on sandbars and streambanks.