Use of mining-contaminated sediment tracers to investigate the timing and rates of historical flood plain sedimentation

Abstract

Changes in land use practices following European settlement in the 1830s produced accelerated sedimentation on virtually all valley floors in the Blue River Watershed, Wisconsin. The contamination of sediments by Pb and Zn mining allowed us to calculate cross-valley rates of flood plain sedimentation for three time periods: the pre-mining period (1830–1900), the mining period (1900–1920), and the post-mining period (1920–1997). Most of the eight valley floors examined contained multiple presettlement surfaces. Significantly higher rates of sedimentation occurred on the lower flood plain surfaces, while the terraces were high enough to prevent sedimentation from most floods. Higher rates of sedimentation on the lower surfaces eventually reduced valley floor relief and, consequently, lateral differences in sedimentation rates. Tributaries and larger valleys downstream exhibited differences in the timing and rates of historical flood plain sedimentation. While rates of sedimentation were high during the pre-mining period in tributary valleys, the lower valleys were receiving little or no pre-mining alluvium. Little pre-mining alluvium was found in mid-basin reaches, suggesting that most of the pre-mining sedimentation was limited to headwater locations. During the mining period, lateral channel migration and the development of meander belts increased the conveyance capacity of tributary and mid-basin channels, which decreased overbank flooding and produced lower rates of sedimentation during the post-mining period. The meander belt channels also had an effect on the lower portion of the watershed by increasing flood magnitudes and the transport of sediment downstream, thereby shifting the locus of sedimentation from the tributaries to the larger, lower valleys. Sedimentation rates in the largest, most downstream site were an order of magnitude higher during the post-mining period than any of the sites upstream.