Effects of Agriculture on Erosion and Sedimentation in the Piedmont Province, Maryland

Abstract

A study of surficial deposits in the Piedmont province of Maryland has made it possible to construct a balance equation of sediment production and deposition since erosive agricultural land use began in the 1700s. Truncated Piedmont upland soil profiles imply approximately 0.15 m of soil erosion. In Western Run, a basin 155 km2, consisting of 85 percent Chester and Manor soils, this amounts to 130 × 10−3 hm3/km2 of eroded sediment. Reservoir sedimentation rates imply that 34 percent of the eroded sediment has been carried out of the system. The rest of the sediment remains in the watershed as alluvium in the upper 1 m of flood plains and as colluvium and sheetwash deposits on hillslopes. Agricultural sediment stored in flood plains constitutes 14 percent of the estimated soil erosion. The sediment was deposited mostly by overbank deposition at rates as high as 1.6 cm/yr. The remaining 52 percent of the eroded sediment occurs as colluvium and sheetwash deposits on hillslopes and as fan-shaped colluvial-alluvial deposits at junctures of headwater tributaries. Wood from such deposits was radiocarbon dated at 290 ± 100 yr. A buried junk pile in a flood plain yielded license plates whose dates imply that after 1925 the dump was buried by overbank deposition at rates as high as those for a basin undergoing urbanization. Piedmont basins less than 26 km2 have statistically longer bankfull recurrence intervals than streams with drainage areas greater than 26 km2. This suggests that since the decline of agricultural land use in the early 1900s, small upland tributaries have adjusted to decreased sediment loads by entrenchment into and erosion of sediment deposited since the initiation of colonial agricultural land use.