ASSESSING HISTORICAL PLANFORM CHANNEL CHANGE: WHITEWATER RIVER BASIN, MINNESOTA, USA

The environmental impact of Euro-American agricultural land use within fluvial systems has long been recognized, particularly in the upper Mississippi River basin (UMRB), USA. Few studies in this region have comprehensively looked at the geomorphic response of river channel geometry and planform change following the introduction of Euro-American agricultural land use. As early as the 1890s-1900s, farmers in the Whitewater River valley (WRV), tributary to the UMRB, noted increased erosion of topsoil and hillslopes. By the 1930s-1940s hillslope erosion and valley sedimentation led to aggradation of the valley floor and dramatically increased flood frequency – burying farmland and entire towns. The primary focus of this research is to characterize historic planform change and compare to changes in valley floor fluctuations assessed previously. We analyze aerial imagery at roughly decadal intervals, from 1938 to 2021, to characterize average planform change. We compare our results to prior work that compiled historic cross-section data recording valley aggradation over a similar time interval. Highest rates of planform change were observed between 1951-1972, likely contributed to by several significant meander cut-offs. This interval of time follows the period of highest sedimentation rates (1855 to 1939) in the WRV and the implementation of soil conservation practices in the 1940s. Planform change was minimal in the most recent intervals between 2003 and 2021. Interestingly, this also occurs during a period of increased valley sedimentation potentially resulting from more intense row cropping practices and a well-noted increase in high magnitude precipitation events throughout the region. The decrease in planform change may be the result of a more entrenched channel form that has adjusted for increased magnitude flows resulting from higher magnitude precipitation events. The overall pattern of decreasing planform change may correlate to conclusions reached by Wood (2023) who suggested improved agricultural land management and soil conversation practices reduced mean sedimentation rates in the WRV until recently. We hypothesize that these conservation practices may be allowing the river to become less erratic and more stable within the Whitewater valley over time. However, the later period of rejuvenated sedimentation may reverse this trend in the future.