Sediment budgets in river networks are notoriously difficult to construct, but can be important for quantifying short and long-term changes to fluvial environments. Adequate sediment supply is critical for in channel, bar, and near-shore ecosystems; too much or too little sediment can be a detriment to biota, including mussels and host fish. Hornbach and others (see http://www.macalester.edu/~hornbach/St.Croix/index.html) have been collecting bed sediment at various locations along the St. Croix River since 1990 in an effort to understand controls on mussel population and diversity. These data, along with suspended sediment data collected by other agencies (e.g., Triplett and others, 2003; Metropolitan Council, 2004), can be used to examine trends in sediment transport and/or sediment delivery to the river over time.
We continued the collection of bed sediment in conjunction with mussel quadrat surveying in summer 2004, and initiated sampling of suspended sediment at various locations along the river. We developed rating curves (relating suspended sediment concentration and mean daily water discharge) for several locations along the St. Croix River, and compared these to historical data. In addition, we examined trends in bed sediment grain size over the last decade. Analysis suggests that while bed sediment grain size is relatively unchanged in most of the study areas, there has been a fining trend in the region below the St. Croix Falls dam. This corresponds with a decrease in the juvenile population of the winged mapleleaf mussel, a federally-endangered species found almost exclusively in this part of the St. Croix (Hornbach, 2005). Future research will address fluvial dynamics and sources of fine sediment in this region. Suspended sediment data are more limited, but suggest transport dynamics may have changed over the last half century.
Dan Hornbach, Biology Department at Macalester College has been making annual summer surveys of mussel populations and bed sediment at various locations along the St. Croix River for the past 15 years (see the web site above). I am interested in constructing a sediment budget for both suspended and bed sediment by looking at active transport. In particular, I am intrigued by sediment fluxes above and below the St. Croix Falls Dam, just upriver from the native endangered mussel populations. Dan Hornbach, Mark Hove and I have submitted a proposal to the National Park Service to focus on better constraining the role of sediment on mussel population dynamics in the area.