ASSESSING THE EFFECTIVENESS OF HISTORIC EROSION CONTROL MEASURES IN WATERSHEDS USING 210PB IN LAKE AND WETLAND SEDIMENTS

Much effort and money has been invested in reducing erosion from watersheds in recent decades, both to protect the land surface and to decrease sediment loading in associated streams. Documenting the effectiveness of these efforts has been difficult. Erosion is often localized in small gullies or at meander bends making it difficult to normalize across an entire watershed. Assessing erosion by measuring sediment loads in streams is also complicated by large spatial and temporal variability and the inherent difficulties in measuring sediments transported in channels. One way of getting around these difficulties is to quantify the rate of sediment accumulation in the lakes or wetlands that ultimately receive sediment-laden runoff. If a lake has not been subject to scour from high flow events or storm activity, the rate at which sediment has been accumulating can be determined using radioisotopes deposited along with the sediment. For sediments deposited within the last century, ²¹⁰Pb is ideally suited with a half life of 22 years. If sediment accumulation has been constant over time, the natural log of excess ²¹⁰Pb activity verses depth will plot as a straight line. If erosion control efforts have been effective, a reduction in sediment accumulation should be apparent as a change in slope in the ²¹⁰Pb data.

Five lakes are under investigation in northwestern Mississippi on the ancestral floodplain of the Mississippi River. This region is under intensive agricultural production, and erosion of fields and subsequent degradation of associated water bodies is a concern. Four of the lakes have had erosion control measures implemented in the last 20 to 30 years, including installation of riparian buffer zones, slotted-board risers, and elevated berms at the edge of fields. Preliminary results from two of these lakes show clear changes in slope for the ²¹⁰Pb data, indicating a 75% reduction in sediment accumulation rate in Beasley Lake (2.0 to 0.5 cm/yr) and an 80% reduction in Moon Lake (1.5 to 0.3 cm/yr). Data from Lake Washington is erratic with no clearly defined trend, possibly indicating significant turbation of these sediments. Results from the fourth lake, Roundaway Lake, are currently in process. As a control, cores taken from an unimproved section of Sky Lake show no change in the slope of ²¹⁰Pb data over the last 50 years.