A Creek Through the Pasture
1Tamarack Farm raises corn, soybeans and a herd of 60 beef cattle on rolling land in Iowa. Brushy Creek flows for 1,000 m through the farm, with a cornfield on one bank and a pasture on the other. The cattle wade into the creek to drink and cool off, and the corn is planted almost to the edge of the bank. Brushy Creek flows into Lake Corwin, which supplies drinking water for the town of Corwin and is a popular place to fish. Seventh grader Joaquín Herrera, whose family owns the farm, helped the county conservation district study the creek for a school project.
2When rain falls on a field, water running off the surface can carry soil and fertilizer into a creek. Soil clouds the water and can bury the gravel where many stream insects live. Nitrate, a form of nitrogen found in fertilizer and manure, can feed large growths of algae downstream, and water treatment plants must remove it when drinking water holds too much. Strips of plants along a creek, called buffers, slow runoff and take up some of the nitrate. Their roots help hold the banks in place, and taller plants shade the water and give birds places to nest.
3The conservation district had already tested three kinds of creek stretches on nearby farms with similar soil. Water samples were taken after the same storm (Table 1).
4The district offered Joaquín's family five plans (Table 2). Plan 5 puts a 30 m strip of trees and shrubs on the cornfield bank and a 10 m grass strip on the pasture bank. The family set two limits: the farm can pay at most $25,000 to build the plan, and it can give up at most 5 hectares (ha) of cropland and pasture. The watershed group that manages Lake Corwin set two goals for the creek below the farm: nitrate of 5 mg/L or less, and at least 13 kinds of stream insects. The family loses about $600 of income each year for every hectare taken out of crops or pasture.