SUSPENDED SEDIMENT AND ORGANIC MATTER ON THE ST. CROIX RIVER AND IMPLICATIONS FOR FRESHWATER MUSSELS

Carl Skarbek, Dan Hornbach, Mark Hove, Kelly MacGregor
Geology, Biology and Environmental Studies Departments, Macalester College, Saint Paul MN

A key characteristic of rivers is the ability to transport sediment and organic matter in the water column. Suspended sediment concentration can affect water quality and therefore benthic habitats, and plays an important role in river morphology. Understanding sources and sinks of sediment over annual to decadal timescales is relevant to quantifying environmental change along the St. Croix River, particularly in the region around the St. Croix Falls dam. Suspended organic material is important for the 40 species of freshwater mussels on the St. Croix River, as they are filter feeders and rely on organic sediment suspended in the water column to obtain nutrients. This project quantifies the flux of suspended sediment and organic matter in the river at four sites during the annual flow cycle. We are interested in the sources and sinks of suspended material between Nevers Dam and Franconia, and seek to understand factors controlling the entrainment and transport of suspended material. We also examine possible correlation between the amount of suspended organic material and mussel population density. Since January 2008 water samples have been taken at four sites along the St. Croix River; Nevers Dam (above the top of the reservoir), National Park Headquarters (in the reservoir), the USGS gauging station (immediately below the dam), and Franconia (2.5 miles downstream from dam). Samples were collected every 1-2 weeks throughout the year. The samples were filtered, dried and weighed to determine total suspended solid (TSS) concentration. The filters were burned and weighed again to determine suspended sediment concentration (SSC), with the difference between TSS and SSC being the total organic content. Suspended organic concentration generally correlates with mean daily water discharge at all four sites, with peaks in the early spring and late summer. Organic material, likely made up of algae and microscopic particulates, makes up 30-60% of TSS at any given time. We calculated the total annual suspended sediment load at each site by calculating the sum of total grams of sediment flux per day (as determined by water discharge) for the entire year. Our calculations show that approximately 2500 metric tons of sediment per year is falling out of the water column and being deposited in the Indianhead Reservoir. This supports bathymetric and GIS analysis showing reservoir deposition exceeding 4 meters in parts of the reservoir over the past 40 years (Loeb and others, 2008). Sites with higher organic material in suspension have lower mussel density, but there was insufficient evidence for us to arrive at any causal relationship. Studies on bedload organic matter (closer to mussel habitat) in addition to TSS sampling at more sites could be useful determining possible trends.

Additional Reading:

SPATIAL AND TEMPORAL VARIABILITY IN SUSPENDED SEDIMENT IN THE ST. CROIX RIVER, MN/WI. Robin Major, Kelly MacGregor, Dan Hornbach, Mark Hove, 2008