TEMPORAL AND SPATIAL VARIABILITY OF SEDIMENT YIELD IN THE BLUE EARTH RIVER WATERSHED, SOUTH CENTRAL MINNESOTA AND NORTH CENTRAL IOWA

Drainage systems in upper Midwestern watersheds continue to evolve. For example, ongoing farm consolidation in this region has promoted increased tillage with expanded artificial drainage networks that rapidly export excess water from tilled lands into nearby streams. Accordingly, the outflow of water and suspended sediment from these rural watersheds is likely changing and warrants ongoing study. The only comprehensive investigation of watershed hydrology in the largely (86%) cultivated Blue Earth River (BER) Watershed, encompassing 4.0 x 10⁵ ha of south central Minnesota and north central Iowa, used monitoring results from a single season, 1996. Our study aims to repeat and amplify the description of this watershed’s discharge and sediment yield by (1) determining equivalent values for these parameters for the 2008 monitoring season, (2) utilizing three new continuous stream gauging stations to more fully define the distribution of flowing water and sediment within the watershed, and (3) using a new, physics-based numerical model to identify cause-and-effect relationships among the various hydrologic factors that influence discharge and sediment yield.

Monitoring results from 2008 show that despite receiving 3% less rainfall, the total discharge of the watershed’s upper 3.9 x 10⁵ ha (97%) was 53% greater than analogous results from 1996. The overall sediment yield for this portion of the watershed increased by 58% in 2008 although yields from the BER’s two major tributaries, Elm/Dutch Creek (1.7 x 10⁵ ha) and East/West/Middle Branches BER (1.8 x 10⁵ ha), decreased by 46% and 0.4%, respectively. Consequently, the sediment yield of the relatively small area (0.4 x 10⁵ ha) that lies immediately north of the confluence of the BER and these two tributaries more than tripled relative to its value in 1996. Model simulations suggest that the sharply increased yield in the northern part of the watershed results from an overall greater stream flow volume and velocity associated with steep, low-permeability soils with up to 40 m of relief, and a doubling of the average channel gradient. We suggest that the basin-wide increase in flow volume and steeper channel gradient in the northern subwatersheds combine to intensify localized erosion and mass wasting that produce the overall elevated sediment load we observe in the BER.