BED SEDIMENT SIZE DISTRIBUTION AND MINERALOGY IN INDIANHEAD RESERVOIR, ST. CROIX RIVER

Karen Jackson, Kelly MacGregor, Dan Hornbach, and Mark Hove
Geology and Biology Departments, Macalester College, St. Paul MN

Dams alter sediment transport dynamics in rivers by acting as a physical barrier to the downstream movement of bed sediment, and by slowing water velocity and allowing suspended material to settle into the reservoir. The St. Croix Falls Dam, installed in 1907, is just upstream of Interstate Park, which is host to a large population of native mussels, including threatened and endangered species. Over the past 20 years Hornbach and others have documented a 90% decrease in the juvenile mussel population at this same location, as well as a gradual decrease in the grain size of sediment at the riverbed there. One hypothesis is that the Indianhead Reservoir above the dam may be a significant source of the fine sediment at Interstate Park. Our goal is to quantify grain size distribution, morphology, and mineralogy of reservoir sediments in an effort to understand river hydrology and sediment transport in the reservoir.

Grab samples and HTH cores were collected along a 10-km stretch of the reservoir between Wild River and the St. Croix Falls dam. A total of 39 samples were collected in summer 2009. Sediment was dried and sieved, and grain diameter distribution and SEM analysis (to determine grain morphology and mineralogy) was conducted. The bed sediment at the top of the reservoir (near Wild River) is composed of moderately rounded spherical felsic grains indicating long distance transport. In addition, there is a smaller fraction of poorly rounded, moderately spherical mafic grains indicating limited transport and closer provenance. Within a kilometer of the dam, bed sediment has a high organic content with some fine spherical sand particles. The sediment is morphologically more angular than sediment further upstream, and has a higher mafic component.

The geometric mean of sediment size distribution decreases from coarse sand near Wild River to fine sand near the dam. Grain size distribution is Gaussian along the majority of the reservoir, although just upstream of the dam sediment size distribution becomes bimodal, indicating an external coarse sediment source is likely. Decreasing sediment diameter demonstrates the majority of the reservoir behaves like a river with respect to water velocity and sediment transport, but the deeper portion nearest to the dam mimics both fluvial and lacustrine environments. The transition between unimodal and bimodal sediment distribution occurs near a deepening of the reservoir, which may suggest a delta is prograding toward the dam. Further work comparing the grain size, composition, and grain morphology of Interstate Park bed sediment and reservoir sediment will improve our understanding of possible sediment transport from the reservoir to mussel beds downstream. Analysis of down-core changes in sediment will allow us to analyze temporal changes reservoir sedimentation.