Sediment, water, change: post-glacial to post-agricultural evolution of river systems in the Upper Mississippi River Valley

Abstract

The Upper Mississippi River Valley (UMRV) experienced considerable change following the Last Glacial Maximum across a wide range of spatial and temporal scales, from region to watershed and from millennium to century. These changes were recorded on the landscape through the continual transport of sediment and water via river systems. In this dissertation, I explore the impact of sediment, water, and change on the UMRV by analyzing sediments from the Whitewater River, a tributary to the Mississippi River in southeastern Minnesota. In Chapter 1, I examine how sediment deposited by water records erosion-rate change by using paired optically stimulated luminescence (OSL) and cosmogenic 10Be sampling of river sediments to build a paleoerosion rate chronology for the post-glacial period. I compare this erosion rate chronology to erosion rates following Euro-American settlement in the region after 1850 C.E. and find that post-glacial erosion rates can be correlated to the advance and retreat of glacial climate and that post-glacial erosion rates are 28–42 times lower than post-settlement erosion rates. In Chapter 2, I study how the routing of water rich in glacially-derived sediment changed in the UMRV during the early Holocene using X-Ray Fluorescence, OSL dating, and Principal Component Analysis of glacially-derived slackwater sediments from the mouth of the Whitewater River. I find a four-phase chronology of meltwater routing down the UMRV via either the Glacial River Warren (Glacial Lake Agassiz) or the Glacial St. Croix (Superior Basin), starting (1) with locally-derived sediments followed by (2) Glacial Lake Agassiz-derived sediments, (3) a period of mixed Glacial Lake Agassiz and Superior Basin sediments, and finally (4) a period of Superior-derived sediments. I relate this chronology to the Marquette Re-Advance of the Laurentide Ice Sheet, thus placing the timing of sediment deposition at the start of the Holocene. Finally, in Chapter 3, I examine how change in base level triggered by aggradation and incision along the Mississippi River during the deglacial period, led to intervals of water incision and sediment deposition along the Whitewater River. I combine detailed river terrace mapping and OSL and cosmogenic 10Be dating to build a glacially-modulated base level chronology for the Whitewater River following the Last Glacial Maximum. With this chronology, I generate field-based dataset demonstrating transport-limited river response to climate-driven base level change. I identify four phases of catchment-wide aggradation and incision: climate driven aggradation from 25–17 ka (Phase 1), incision triggered by base level fall from Glacial Lake Agassiz floodwaters at ~13.4 ka (Phase 2), slackwater deposition and aggradation from glacial meltwater inundation at ~11.6 ka (Phase 3), and a period of deep incision from ongoing meltwater inputs at ~10.6 ka (Phase 4). This dissertation provides new insight into the post-glacial to post-agricultural evolution of the UMRV and relates findings from these studies to broader themes relating to how rivers respond to changing climate versus anthropogenic influence (Chapter 1), how Laurentide Ice Sheet-derived glacial meltwater was rerouted during the deglacial period (Chapter 2), and how real-world, transport-limited rivers respond to base level change (Chapter 3).