New insights into the fluvial geomorphology and its effects on paleofloods in the Black Hills, South Dakota

Abstract

Identification and characterization of a relict landscape in the Black Hills of South Dakota has provided insight into the postuplift history (~30 Ma to present). A new watershed decomposition technique has been developed that utilizes high-resolution digital elevation models and traditional long valley profiles to identify the location and extent of the relict landscape as well as its geomorphic characteristics. Results identified an expansive relict landscape at elevation in the center of the Black Hills that contains fluvially mature features such as low-gradient channels and broad flood valleys. Moreover, the relict landscape and associated watersheds that drain it are undergoing active fluvial incision via knickzone migration. This observation suggested that the Black Hills were once much more subdued than today. At present, channel confinement, lack of potential floodwater storage, and steepened stream gradients exists in reaches of active incision downstream of the relict landscape. Large paleofloods influenced by these processes have been well documented and have been restricted to the narrow and steep canyons that drain the relict landscape. Therefore, active fluvial incision into the Black Hills influences the ability of more recent (Holocene) large floods to leave substantial evidence of their occurrence inside incised canyon walls and should also be considered when studying watershed factors influencing large flood events on the flanks of the Black Hills.