Pollution Agricultural runoff from farms and ranches in the Snake River Plain and many other areas has severely damaged the ecology of the river throughout the 20th century. After the first irrigation dams on the river began operation in the first decade of the 20th century, much of the arable land in a strip a few miles wide along the Snake River was cultivated or turned to pasture, and agricultural return flows began to pollute the Snake. Runoff from several feedlots was dumped into the river until laws made the practice illegal. Fertilizer, manure and other chemicals and pollutants washed into the river greatly increase the nutrient load, especially of phosphorus, fecal coliforms and nitrogen. During low water, algae blooms occur throughout the calm stretches of the river, depleting its oxygen supply. Much of the return flows do not issue directly back into the Snake River, but rather feed the Snake River Aquifer underneath the Snake River Plain. Water diverted from the river for irrigation, after absorbing any surface pollutants, re-enters the ground and feeds the aquifer. Although the aquifer has maintained its level, it has become increasingly laced with contaminants. Water in the aquifer eventually travels to the west side of the Snake River Plain and re-enters the river as springs. Throughout much of the Snake River Plain and Hells Canyon, excessive sediment is also a recurring problem. In December 2007, the U.S. Environmental Protection Agency (EPA) issued a permit requiring owners of fish farms along the Snake River to reduce their phosphorus discharge by 40%. Pollutant levels in Hells Canyon upstream of the Salmon River confluence, including that of water temperature, dissolved nutrients, and sediment, are required to meet certain levels. Discharge The Snake River's average flow is . The United States Geological Survey recorded the river's discharge from a period of 1963–2000 at a stream gauge below Ice Harbor Dam. In that period, the largest average annual flow recorded was in 1997, and the lowest was in 1992. The lowest recorded daily mean flow was on February 4, 1979. On August 27, 1965, there was temporarily no flow as a result of testing at Ice Harbor Dam. The highest recorded flow was on June 19, 1974. The highest flow ever recorded on the Snake River was at a different USGS stream gauge near Clarkston, which operated from 1915 to 1972. This gauge recorded a maximum flow of