12.2.3: Off-Site Effects

The soil removed from fields has significant negative effects off the farm as sediment accumulates in streams, rivers, reservoirs, and estuaries, or blowing dust reaches towns and cities. In fact, sediment remains the number one contaminant for most waters around the world, and it often also carries other pollutants like nutrients, pesticides, and other chemicals. From a purely economic perspective, the off-site costs of soil erosion affecting fisheries, recreation, and industry can be greater than the lost agricultural productivity, especially if the receiving waters are used by many people. Many conservationists argue that any amount of erosion is unacceptable, even when losses are less than T. This is a good point, as small amounts of soil can have an outsize impact on water and air quality, meaning that soil losses less than T may be tolerable in terms of agricultural productivity but not in terms of environmental quality. This is especially the case when dealing with soils that are high in clay content, where the particles become suspended as colloids in runoff water. The particles do not settle out in ponds or filter strips and can be transported long distances from the source, along with nutrients and pesticides. Similarly, clay and silt particles suspended in the air can be transported long distances and can cause respiratory problems.

When soil washes (rain-caused) or blows (wind-caused) off a field it has downstream effects. In the humid Midwest, soil enters drain tiles, ditches, and streams and in short order enters the nearest river or lake. There are huge problems associated with soil becoming a pollutant in lakes. Not only do the lakes become cloudy and unpleasant for swimming, but the turbidity clouds the amount of light upon which many water plants depend. The plant ecology changes, which in turn changes which creatures can live in the water from crayfish to mollusks to small fry fish. Cloudy water also lowers the amount of dissolved oxygen in the water which affects which fish species can live there.

Sediment (silt) covers lake bottoms and grassy vegetation coating them and preventing ribbons of fish eggs from attaching and hatching. The formerly abundant perch population in Green Bay has crashed. Only one commercial fisherman now holds a license on the Bay. Of course, farming is not the only reason the fishery has collapsed, but it is a very big reason. Perhaps it does not make sense to have a million dairy cattle in the region, one of the largest concentrations in the world, in this close of proximity to a treasure like Green Bay and Lake Michigan.

Figure \(\PageIndex{1}\): Sediment covering a sea grass bed. Source: FWC : Creative Commons

Erosion: A Short-Term Memory Problem?

It’s difficult to fully appreciate erosion’s damage potential because the most severe erosion occurs during rare weather events and climate anomalies. Wind erosion during the Dust Bowl days of the 1930s, which resulted from a decade of extremely dry years, was especially damaging. About one-third of the water erosion damage that occurs in a particular field during a 30 year period commonly results from a single extreme rainfall event. Like stock market crashes and earthquakes, catastrophic erosion events are rare, but the impacts are great. We must do our best to understand the risks, prevent complacency, and adequately protect our soils from extreme weather events.