The Merced Irrigation District (MID) operates most of the irrigation infrastructure on the lower river, which supplies water to of farmland. As a whole, the system includes about 4,000 control gates and of canals and laterals. Irrigation has taken most of the water out of the lower river, which is often reduced to a small stream where it joins the San Joaquin. Irrigation return flows carry pesticides, fertilizer and other pollutants into the river. The MID is federally required to allow at least of water annually to flow continually down the river, not including floodwater releases.