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Title: LONG-TERM GROWTH OF A VALLEY-BOTTOM GULLY, WESTERN IOWA

Author
item THOMAS, JOHN - IOWA STATE UNIVERSITY
item IVERSON, NEAL - IOWA STATE UNIVERSITY
item Burkart, Michael
item Kramer, Larry

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2004
Publication Date: 8/1/2004
Citation: Thomas, J., Iverson, N., Burkart, M.R., Kramer, L.A. 2004. Long-term growth of a valley-bottom gully, western Iowa. Earth Surface Processes and Landforms. 29(6):995-1009.

Interpretive Summary: Gully erosion in small streams is a problem associated with row crop agriculture. Gully expansion reduces the cropland area and the resulting sediment is a major source of stream contamination. Answering questions about the causes of gully expansion will help design management systems that reduce the rate of gully growth. A study in western Iowa used 36 years of sediment measurements and land-surface surveys to determine if stream processes or bank-failure processes were dominant in explaining gully expansion. It was found that bank failure closely correlated with stream flow following storms when data were examined over a year or decade. However, during individual storms, particularly in April and May, sediment produced by bank failure was equal to the sediment removed by the stream. This allowed widening of the gully as the slopes adjusted to the amount of sediment removed. Widening of the gully by 20% was correlated with terrace construction in the watershed and increased groundwater flow. These results will help farmers and conservation agencies design land management practices that change the balance of groundwater and surface flow in order to reduce the rate of gully growth.

Technical Abstract: A study of the relative importance of fluvial sediment transport and mass wasting processes on gully growth was conducted using 36 years of ground-surface surveys and suspended sediment discharge measurements in the Deep Loess Region of western Iowa. From 1964 to 2000, the volume of the gully increased by 9000 m**3, with headward advance of 100 m and an average of 340 tons of sediment removed each year. Volumetric growth rates were significantly related to sediment discharge following storms when examined over annual to decade time scales. This indicates that gully growth was limited by fluvial transport capacity rather than the supply of sediment from mass wasting. However, during some months and during individual storms, growth rates were limited by colluvium supply, indicating that rates of sediment production by mass wasting and rates of sediment removal by the stream in the gully were nearly in balance. This equilibrium was maintained by the tendency of gully walls to adjust their slopes to the rate of colluvium removal by the stream. This balance also indicates that much of the sediment load in the stream was derived from mass wasting rather than sheet and rill erosion following some storms. Widening of the gully by 20% since 1980 was caused by a reduction in lateral gully slopes and was correlated with terrace construction in the watershed and increased baseflow, which may have decreased slope stability.