Wildfires affect rivers and streams in many ways. The health of a river or stream is a reflection of the condition of the surrounding watershed, and severe fires dramatically alter the surrounding watershed. An intense wildfire can change a river or stream rapidly by altering the hydrology, water quality and aquatic communities. Over time, the river environment will recover and can shift to a new and different equilibrium.
Wildfire changes the hydrology of a stream by altering the flow of water. Large amounts of burnt debris from trees and vegetation can end up in a stream, and this debris changes the speed and direction that water flows. There are fewer trees and plants on the landscape following a fire, so during rain, there will be more runoff into the stream. This combination of debris and excess runoff can alter the stream channel, and the locations of runs, riffles and pools within a stream may not be the same after a fire.
Wildfires can impact water quality in streams in multiple ways. Because vegetation is removed from the landscape in a fire, erosion increases and streams and rivers will have increased amounts of sediment in the water. After intense fires, it may take decades for sediment accumulations to return to pre-fire levels. Wildfire also affects water chemistry, and immediately following a fire, nutrient levels (nitrogen, phosphorus, potassium) can increase significantly. These nutrients are used by algae, and the excess nutrients can lead to population explosions of algae called algal blooms. These algae can decrease light penetration to the bottom of the stream and can lead to decreases in dissolved oxygen, which is a critical component for the survival of aquatic communities.
Aquatic communities of insects and fish are often altered following a fire. Because there are fewer trees in the watershed, insect communities may shift from insects that feed on leaf litter to insects that feed on algae, and increased sediment in the water may cause shifts to sediment-tolerant insects. In severe cases, fish populations can be completely wiped out by fire. If there are no major barriers such as dams or poorly designed road crossings, live fish communities upstream and downstream will repopulate the wildfire-impacted areas. Some fish, including many species of trout, only tolerate a small range of temperatures. If the fire burns the tree canopy over a stream, the lack of shade and extra sunlight could increase water temperatures to the point that certain fish species can no longer occupy the stream. The aquatic community may eventually reach a stable equilibrium, but this process can take decades following severe fires.