LANDSCAPE EVOLUTION AND STREAM TERRACES OF THE WHITEWATER RIVER VALLEY, MINNESOTA: PRELIMINARY RESULTS

The upper Mississippi River basin (UMB) is a transient fluvial system where landscapes reveal ongoing response to episodic base level-fall following the last glacial maximum (LGM). Sediment-poor meltwater discharge during deglaciation incised into the glacial-stage floodplain – the present-day Savanna Terrace – lowering base level for tributary drainages. Tributary drainage networks continue to incise in response. Building on recent research within the UMB, our objective is to understand fluvial-system response to base-level fall by reconstructing the landscape evolution of the Whitewater River valley (WRV) and compare our results to prior studies on other tributary systems within the UMB. The WRV serves as an ideal natural laboratory, unlike those previously studied, as it did not experience glacial meltwater input during the LGM. Thus, base-level control is the primary driver of incision. Geochronologic (OSL), geophysical (GPR), and geospatial methods will be used to understand the spatial distribution of stream terraces, date and interpret terrace fill, and reconstruct longitudinal profile and drainage-network evolution.

Presented here are preliminary results from our terrace mapping. To map stream terraces of the WRV, we utilize 1-m LiDAR-derived digital elevation models (DEMs) that are transformed into hillshade imagery to identify terrace treads. Our preliminary results reveal that more than 30 m of incision has occurred in this watershed. Incision is transient in the modern landscape, with a knickzone observed in the longitudinal profile ~40-65 km upstream of the confluence. This knickzone may be a compound feature resulting from post-LGM incision as well as exhumation of a preexisting knickpoint from some period prior to the Wisconsin glaciation. A deeply incised valley exists downstream of the knickzone with a high prominent terrace present, the Minneiska Terrace, which is likely equivalent to the Savanna and Wissota terraces of the Mississippi and Chippewa Rivers. Very few terrace remnants exist lower than this high terrace, but some can be observed in the tributary drainages. Within the knickzone, more sporadic remnants exist at a variety of elevations, indicative of progressive and ongoing incision. Upstream of the knickzone, the valley transitions into a low-relief upland surface.