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200,000 o o 100,000 o ~ -< ~ ~ ~ ~ ~ ~ '" ~ ~ ::3 10.000 " ~ ~ ~ ~ ~ z " ~ . o Uc ) :;: z " ~ u ~ " " ~ ~ ~ z ~ ;!! " ~ ~ 1000 EXPLANA TION o Urbanized (data from Wolman, 1964) x Mostly natural (U. S. Geological Survey data) I 100 50 0.01 o Little Falls Branch x~o I ~ o A x x Watts Branch x- :;-i x x x x x 10 100 0.1 DRAINAGE AREA, IN SQUARE MilES I l Figure 7.-Annual sediment production per. square mile for urbanized and natural areas. Zones: A, agricul- tural; C. under construction; VC, under construction and undiluted. going building construction, but highly diluted before reaching channels; and UC, undiluted sediment yields delivered to stream channels from construction sites. They found that when building sites are de- nuded for construction, excavations are made, and dirt is piled without cover or protection near the site, the sediment movement in a rill or stream channel is very large in terms of tons per year immediately downhill from the con- struction site. If the channel contains little water except during storms (an ephemeral stream), there is no chance for dilution; during storm flow the sediment movement is great. If the construction debris gets into perennial channels, or for other reasons is distributed along a channel or dispersed over a wide area, the dilution lowers the yield per square mile per year. Thus, Wolman and Schick drew the distinction between agricultural, construction, and construction-undiluted. For very small areas, Wolman (1964) said, "Because construction denudes the natural cover and exposes the soil beneath, the tonnage of sediment derived by erosion from an acre of gronnd under construction in developments and highways may exceed 20,000 to 40,000 times the amount eroded from farms and woodlands in an equivalent period of time." Figure 7 shows the data as a relation between annual sediment yield per square mile and drainage basin size. The usual suspended load station is on a basin of more than 10 sq mi in area. Seldom is urbanization complete f. basins of this size. , . ; 12