Atmospherically deposited radioisotopes have been used to
discriminate between upland field erosion versus streambank erosion as
contributors to riverine suspended sediment. These traditional fingerprinting methods are limited
by the spatial variability and correction factors necessary to accurately
characterize the upland (or agricultural field) source fingerprint. To get around these obstacles, this
study utilized excess 210Pb in the sediment of seepage lakes, with
no channelized inputs, as reference systems to ascertain a temporally and
spatially integrated upland field fingerprint, encompassing sheet, rill and
gully erosion. The relationship
between 210Pb flux and modern sedimentation rate was plotted for 15
such reference lakes in the Minnesota River watershed. The slope of this relationship, 4.8
pCi/g, (p<0.001, r2=0.58) predicts the average activity of 210Pb
on sediments eroded from upland fields. The relationship of 210Pb
flux to sedimentation rate was also applied to 20 cores collected from Lake
Pepin (Mississippi River, MN and WI).
Lake Pepin receives greater than 85% of its sediment burden from the
Minnesota River and acts as a basin wide integrator of bank and upland erosion
sources. A plot of 210Pb flux to sedimentation rate at two core
intervals, pre 1965 and 1965 to 1996, yielded slopes of 3.16 and 1.87 pCi/g
respectively (r2 > 0.94, p<0.001). The ratio of these slopes to the slope determined in the
reference lakes predicts the proportion of sediment derived from upland
sources. This comparison estimates
that the relative contribution from upland sources to Lake Pepin has decreased
from approximately 65% to 38% in the past 30 years. During this period, the overall sediment accumulation rate
remained 8 to 10 times greater than historic rates, thus highlighting the
significant contribution (>60%) from non-upland sources to the current
sediment loading.