Impact of hydrologic change on the riparian vegetation dynamics of the Minnesota River basin
Riparian vegetation plays a key role in ecological, hydrological and sedimentary processes in streams. In the Midwestern United States, there has been substantial change to streamflow regimes from land-use change, increased sub-surface drainage and precipitation increases. Changes in high flows may have very different ecological impacts than changes to base flow or mean annual flows. In particular the impacts of prolonged moderately high flows in summer may reduce tree establishment and survival altering plant community composition and structure over time. Our main research questions included “How has hydrologic change effected establishment and growth of riparian woody species on the Minnesota River (MR) (44,000 km2 watershed area) and a smaller tributary, Elm Creek (EC) (700 km2watershed)”. Secondly we asked how riparian vegetation change may influence stream width. Field data collection included assessment of plant species frequency and cover, analysis of sediment particle size and deposition on sandbars. Hydrologic analysis included use of the Indicators of Hydrologic Alteration (IHA) to quantify flow change metrics. Sandbar submergence frequency and duration were estimated using cross-sectional surveys and stream flow data. Mapping of sandbars was done along the Minnesota River in GIS to estimate recent vegetation establshment remotely.
Sandbars on the MR consisted of 85-90% sand by particle size within the top 60cm. There was less sandbar area in the lower MR near St. Paul than the upper MR. On both rivers, sandbar willow (Salix interior) had the highest frequency and coverage, exceeding silver maple (Acer saccharinum) and cottonwood (Populus deltoides) by a factor of 8-9x on EC and 22-179x on MR. The MR sandbars had a lower percent of vegetative cover than EC overall. IHA data analysis showed that mean annual flow in the MR increased by 75% since 1980. Consequently the duration of sandbar submerging flows has increased by 25 days during that same time period decreasing the time and space available for woody plant establishment and growth. Consequently less of the sandbar area is vegetated potentially increasing sediment mobilization at high flows and leading to wider rivers. Overall our research suggests that increased volume and duration of summer streamflow has impacted vegetation composition in these two rivers, shifting it towards shrubs rather than trees. Further research is needed to determine the effects of these plant composition shifts on sandbar growth and river width change in rivers that have experienced recent flow increases.