CHANGE IN SEDIMENT ACCUMULATION IN THE LOWER MINNESOTA RIVER

Ever since glacial Lake Agassiz formed the glacial River Warren spillway, the modern river occupying the valley, the Minnesota and its tributaries have been filling it in. The rate of sediment accumulation varies spatially, with climate, and other factors that affect watershed hydrology—namely, ground cover and artificial drainage.

Peak flows and in-channel sediment loads have increased, rivers widened, and nick points retreated more rapidly since the intensification of agriculture. A riverine lake on the Mississippi River downstream of the Minnesota confluence, Lake Pepin, is filling in almost ten times faster than pre-settlement rates. The watershed district charged with maintaining a navigation channel in the lower Minnesota River between Chaska and Minneapolis would like to document how sediment accumulation rates have changed in the reach that they manage.

We chose two floodplain lakes that are in stable locations behind the levee. A transect of 6 cores from each allowed for a complete record of sediment and vegetation change using fossil pollen and non-pollen palynomorphs (stomata, algal cenobia, fungal spores and charcoal particles). Pollen stratigraphy from dated cores in non-floodplain locations were correlated to these floodplain-lake cores leading to estimates of changes in accumulation rates of sediment deposited in overbank settings along the lower Minnesota River.

Ongoing core description and correlation suggest that cores 1 meter in length did not capture the Ambrosia rise indicative of initial European settlement but did capture a distinctive oak rise in the 1950s suggesting sedimentation rates of approximately 1.5 cm/year for the period in which the most pronounced changes in sedimentation to Lake Pepin were documented. The sedimentation rates will be further refined and linked to land-use change in the watershed.