PS 34-20
Ecohydrological investigations of Minnesota River sandbar vegetation
Over the past thirty years, many upper Midwestern agricultural watersheds have experienced increased stream flow volume and altered flow regimes resulting in hydrologic and geomorphic changes. Increased summer flows in particular have led to a decline in success of vegetation colonization on sandbars by reducing the area and duration of exposed sand surface. Decreased channel stability and increased erosion rates are the result, both of which contribute to the sediment load. The study site, the Minnesota River, has widened by 52% since 1938. To assess the role of riparian vegetation on channel widening and sediment loading, data to characterize riparian vegetation, bank elevation, and soil particle size profiles were collected. This data included vegetation inventories and soil samples collected on a transect from the water line to the mature tree line as well as elementary bank elevation surveys using a leveling rod. Five sandbars along the Minnesota River were investigated. The soil samples were assessed for particle size distribution. The soil particle size data were compared to the vegetation data to ascertain whether transportable-sized sediment was being deposited in the zones of dense vegetation.
Results/Conclusions
Vegetation transect data showed that new woody growth was confined to narrow belts high above the stream. Sandbar willow was most abundant with 100% relative frequency. Cottonwood had a 20% relative frequency. The sites with low bank elevation gradients from the water line to tree line were found to have particle size profiles consisting of 56% coarse and medium sand. Steeper banks had soils composed of 75% fine-very fine sand. The sites with flat elevations also exhibited little or no riparian vegetation colonization. These findings demonstrated the lack of successful vegetation colonization on flat sandbar areas, most likely as a result of insufficient time to colonize on bare soil. Vegetation is thought to induce sediment deposition and point-bar growth, however flat sandbars where this usually occurs were found to be essentially bare of vegetation. Most of the woody vegetation colonization occurs in the early summer months and little of this was observed as there were many high flow events in the river relatively late in the season, primarily at the end of May through July. Reduced woody plant colonization reduces point-bar growth contributing to channel widening on the lower Minnesota River.
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