Doctoral Dissertations

Date of Award

5-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geography

Major Professor

Carol P. Harden

Committee Members

Sally Horn, Kenneth Orvis, Michael Clark

Abstract

Because of the potential for sediment storage in watersheds over a variety of timescales, and because the mobilization of stored sediment is related to the frequency-magnitude characteristics of runoff events, short-term sediment monitoring data cannot be assumed to apply beyond the period of measurement. Medium-term (101 -102 years) sediment yield data are required to understand the dynamics of sediment storages as well as the persistent effects of natural and anthropogenic watershed disturbances. In this dissertation I present a 29-year sediment yield record for a small watershed in Virginia derived from analyses of reservoir sediment. The sediment yield record is evaluated with regard to precipitation and discharge data from nearest monitoring stations and to dendrohydrological evidence of past flooding within the catchment. The sediment yield reconstruction was accomplished by correlating stratigraphic layers within a network of lake sediment cores, calculating the mineral mass of each layer, and establishing an absolute chronology for one representative master core in the network. Measurements of magnetic susceptibility, loss-on-ignition and radionuclide content of one-centimeter sediment increments were used to accomplish this reconstruction. The resulting sediment yield record isolates four broad periods of sediment yield as well as sediment contributions from two individual large storm events. Average values of sediment yield for each of the primary sediment yield periods appear related to annual precipitation characteristics at the site. I use the storm event sediment yields and the 29-year average to estimate the relative contributions of large infrequent floods and frequently occurring moderate floods to medium- and long-term sediment yield. The sediment yield contribution from the largest storm event forms only 10% of the total 29-year sediment accumulation, and much less when the recurrence interval of the associated flood event is accounted for. This importance value is similar to that published for much larger streams in the area, and therefore does not support the theory that the importance of sediment yield contributions from high-magnitude events increases as basin size decreases.

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