A strip of unmown vegetation left along a stream bank, called a riparian buffer, does more work than its modest width suggests. Roots hold the bank together during floods, when bare soil would simply wash away. The plants filter runoff before it reaches the water, catching fertilizer and sediment that would otherwise cloud the stream and feed algae blooms downstream. Shade from the buffer also keeps the water several degrees cooler, which matters because many fish species cannot survive water above a certain temperature. Farmers are sometimes paid to leave these strips uncultivated, a small loss of tillable land traded for a measurable gain downstream.