Over the last 100 years, about 40 percent of the topsoil in the dryland-farming Palouse subunit has been lost because of erosion [23]. Most soil erosion in this region is caused by storm runoff. In the Quincy-Pasco subunit where irrigated farming predominates, most soil erosion is caused by runoff of excess irrigation water from cropland.
Even though most organochlorine pesticides (DDT, for example) are no longer used, they break down slowly and are still present in the environment. Because they bind strongly to soils, they are carried with eroded soils into streams. The most persistent breakdown product of DDT, p,p'-DDE, was found in all parts of the Study Unit except in the headwaters of the Palouse River Basin and upper Crab Creek (fig. 28). Concentrations of p,p'-DDE exceed guidelines for the protection of aquatic life [24] at 22 percent of the sites sampled [25]. Other organochlorine pesticides found in streambed sediments at concentrations exceeding guidelines were heptachlor epoxide, dieldrin, and Lindane (fig. 29).
Polychlorinated biphenyls (PCBs) are organochlorine compounds that were detected only at sites sampled downstream from the cities of Moscow and Pullman in the Palouse subunit [25]. PCBs, which were widely used in electrical transformers, have not been manufactured in the United States since the 1970s because they are toxic and persistent in the environment.
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Figure 28. p,p'-DDE concentrations in streambed sediments commonly exceed the guideline for protection of aquatic life [24] in dryland farming areas.
Figure 29. Concentrations of organochlorine compounds in streambed sediments sometimes exceed guidelines for the protection of aquatic life [24].
Furrow irrigation causes more erosion than sprinkler or drip irrigation [26]. DDT is carried with eroded soils [27], and highest concentrations of DDT in streambed sediment and fish tissue were detected in watersheds with more furrow irrigation (fig. 30).
Figure 30. DDT concentrations in streambed sediment and fish increase as the percentage of furrow irrigation increases.
Beginning in the 1970s, the use of sprinkler irrigation in the Quincy-Pasco subunit increased; with this change in irrigation method came reports of reduced erosion. A good indicator of the severity of erosion in a drainage basin is the concentration of suspended solids in streams. Figure 31 shows the relation between irrigation method and average daily yields of suspended solids discharged by Lower Crab Creek to the Columbia River.
Figure 31. Average daily yields of suspended solids discharged by Lower Crab Creek to the Columbia River decreased from 1975 to 1988, coinciding with a decrease in the use of furrow irrigation [28, 29].
Data collected from 1993 to 1995 also show the relation between irrigation method and erosion. For nine drainage basins sampled in 1994, average daily yields of suspended sediment (which is collected and analyzed differently than suspended solids) ranged from 0.4 pound per acre from a basin with no furrow irrigation to about 20 pounds per acre from a basin where about 60 percent of cropland is irrigated by the furrow method [28, 29].
Studies by the U.S. Department of Agriculture indicate that improved farming practices that began in the 1970s have reduced erosion of soil from cropland in the United States by up to 25 percent [30]. Field observations and studies indicate that a reduction in erosion of about the same magnitude has occurred in the Palouse subunit [31]. The decrease in suspended sediment discharged by the Palouse River may be indicative of less erosion (fig. 32). However, the average sediment concentration for the period from 1962 to 1971 is skewed by a very wet year (1963), which did not occur in 1993-96. Therefore, the difference between the two periods is probably exaggerated on the graph.
Figure 32. The average sediment load per unit volume of water discharged by the Palouse River has decreased (comparing 1993-96 with 1962-71 data).
Because trace elements bind to sediments, and sediments accumulate on the streambed during low flows, analysis of streambed sediments is a good way to relate concentrations to the effects of land use. In the Central Columbia Plateau, streambed sediments are derived mostly from eroded soils. If the concentration of a trace element in streambed sediments is greater than the concentration in undisturbed soils, a source related to land use is probable. However, most trace element concentrations in streambed sediments were not commonly above concentrations in undisturbed soils of the Study Unit (fig. 33) [32].
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Figure 33. Except for selenium, concentrations of trace elements in streambed sediments and soils were not commonly above concentrations in undisturbed soils in the Central Columbia Plateau.
