128-7 The Avulsive Failure of the Rapidan Dam, MN, USA: A Case-Study of Modern Spillover-Style Transverse Drainage Development and the Corresponding Environmental and Societal Implications

Abstract:

The Blue Earth River (BER) basin, Minnesota, USA, has undergone a conversion from prairies and wetlands to extensive row crop agriculture and artificial drainage over the last two centuries. This has resulted in well-documented geomorphic change accompanied by increased sediment and nutrient loading which is further exacerbated by changing precipitation regimes. The Rapidan Dam (RD) was constructed on the BER in 1908-1910, ~19 km upstream from the confluence with the Minnesota River. RD represented a ~16 m local base level high, resulting in rapid sedimentation within the reservoir that was near capacity at the time of failure. On June 24, 2024, following ~20 cm of rainfall over three days, the BER spilled around the western edge of RD, causing an avulsive failure. Media coverage and observations of the event provided a unique opportunity to investigate fluvial response to both dam failure and abrupt base level change. As the failure evolved, it became evident that this was akin to natural spillover processes that create transverse drainages – a fundamental mechanism in reorganizing drainage networks.  

Over the past ~2.5 years, repeat LiDAR, structure-from-motion surface models, and cross-sectional transects were used to quantify the geomorphic response of the BER. The rapid lowering of base level following the event triggered the development and migration of knickpoints through underlying alluvial, glacial, and Paleozoic sedimentary strata, with some traveling upwards of 130 m hr^-1. These knickpoints excavated 1.6 million m³ of hillslope and reservoir sediment within 2 km upstream of the dam. The downstream portion now exhibits near-braided morphology as large volumes of remobilized sediment move through the system. Longitudinal profile analysis indicates that the BER continues to approach a graded profile, but the majority of this adjustment occurred within one week of spillover. The result of this event is the transverse gorge through which the BER now flows. 

We suggest this ongoing study is a powerful example of avulsive-style dam failure and transverse drainage development that can aid both geomorphologists and municipalities. The potential for this type of event is increasing with changing hydroclimatic regimes and aging infrastructure, driving the need to better understand the geomorphic implications of these events to protect people and the environment. This study also exemplifies the unprecedented geologic impact human activities have on landscape evolution. 

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