Characterization of Reference Suspended- Sediment Transport Rates for Level III Ecoregions of Minnesota""

As a result of 'natural' and human-induced changes to precipitation, runoff production and land use water-quality issues due to excessive erosion and transport of suspended-sediment are a primary concern throughout the surface waters of Minnesota. Within Minnesota there are a number of Level III Ecoregions that are the subject of this investigation and report: Ecoregion 46, Northern Glaciated Plains; Ecoregion 47, Western Corn Belt Plains; Ecoregion 48, Lake Agassiz Plain; Ecoregion 50, Northern Lakes and Forest, and Ecoregion 51, Northern Central Hardwood Forests. Rapid Geomorphic Assessments carried out at current and historical USGS gaging stations found that more than half of sites visited in Ecoregions 46, 50 and 51 were considered stable, while the majority of sites in Ecoregion 47 and 48 were found to be unstable. Calculated suspended-sediment transport yields (load per unit area) at the 'effective discharge' (the flow that occurs, on average, once every 1.5 years or Q1.5) and the mean annual yield were sorted into stable and unstable groups based on channel stability information. "Reference" suspended-sediment transport values were thus obtained from analysis of the distribution of suspended-sediment yields for streams determined to be geomorphically stable at the time of suspended-sediment sampling. At the Q1.5 discharge, "reference" yields range over two orders of magnitude; from 0.0039 T/d/km2 for Ecoregion 46, to 0.48 T/d/km2 for Ecoregion 47. Mean-annual "reference" suspended-sediment yield values also varied by two orders of magnitude, from 0.351 T/y/km2 to 20.3 T/y/km2 also for Ecoregions 46 and 47, respectively. Statistical analysis of the differences between stable and unstable sites within each Level III Ecoregion were significant at the 0.05 probability level in all cases except Ecoregion 51 Q1.5 yield values. However, stable and unstable values were statistically significant with the removal of two major outliers. This provides support for the overall approach of determining "reference" or 'target' rates for suspended-sediment transport and the development of TMDLs in Minnesota.