THE SOURCES AND AGES OF FINE-GRAINED SEDIMENT USING ELEMENTAL ANALYSIS AND FALLOUT RADIONUCLIDES FOR AN AGRICULTURAL STREAM, WALNUT CREEK, IOWA

Determining the sources and age of fine sediment in transport and in channel and floodplain storage is important for understanding the transfer of sediment though the watershed and for managers determining the time scales over which actions to reduce sediment will be effective. In the agricultural Walnut Creek watershed, Iowa, the sediment elemental-fingerprinting approach used on 8 samples collected during the summer of 2013 indicated sediment sources as follows: cropland 62%; streambanks 36%; and prairie, pasture and unpaved roads each contributing <=1%. The surface-derived portion of sediment were dated with ²¹⁰Pb_ex, which provides ages from ~1 to 100 years, and ⁷Be which can date sediment to within a year. The surface-derived portion of sediment (primarily cropland) from the 8 events had ages ranging from 5 to 52 years (mean 30 years) using ²¹⁰Pb_ex. Some of the surface-derived sediment was quite young, as indicated by ⁷Be ages ranging from 23 to 202 days (mean 127 days). The ²¹⁰Pb_ex ages indicate that much of the surface-derived sediment is in channel storage for decades. A survey of sediment in channel storage, along with literature values, support a large component of sediment in channel storage, ranging from 50% to 100% of the annual suspended-sediment load (16,000 Mg/yr). Based on studies from other watersheds, we speculate that sediment deposited on the floodplain during overbank events may reside there up to 1000s of years until the channel erodes the sediment. We depict a geomorphic model where surface-derived agricultural sediment is delivered to the channel system and subsequently conveyed to the watershed outlet along with channel bank sediment at three time scales: 1) a geologic-millennial time scale, 2) a time scale of decades, and 3) young time scale (<1 year).