Fire is a natural process in many forests and prairie environments that allows these ecosystems to undergo vegetative succession. In fact, many plants are adapted to withstand fire conditions, and either colonize quickly after a burn, or have special adaptations that require intense heat in order to release seeds. Once a balanced process, and an important part of a healthy ecosystem, natural fire regimes can provide improved aquatic habitat through increased channel complexity.
However, a century of fire suppression policies throughout the U.S. combined with a rapidly-changing environment, have altered natural fire regimes, resulting in higher-intensity, and more frequent wildfires. Larger, catastrophic fires are more commonplace because of higher fuel load (dead logs, ladder fuels, brush, etc.) that would have otherwise been consumed with light, low-intensity ground fires. The once rejuvenating effects of a more moderate fire regime with an abundance of habitat refuges, has been hindered in many places by fire suppression, damming, and many other impacts of development.
How does fire impact streams?
Immediately following a wildfire event, ash and burnt organic matter fall into the water, raising nutrient concentrations and using-up dissolved oxygen. Water may be blackened, cloudy and scummy from the ash, affecting visibility for fish and possibly clogging gills. During this period fish kills are possible Reptiles, frogs and river mammals that have survived the fire may also be further disturbed during this period. If fire suppression was attempted, fire suppressant chemicals that have entered the stream may cause fish kills or affect nutrient concentrations. Extreme heat can cause soils to become hydrophobic, reducing water infiltration, increasing runoff, and limiting natural regeneration of vegetation.
Short-term effects of fire on streams and rivers include an increase in the amount of light reaching the stream, which promotes changes in fish, insects, plants, and algae. Increased water temperatures and greater fluctuations in temperature ranges reduce dissolved oxygen. Reduced ground cover and streamside vegetation mean less filtration capacity and higher amounts of sedimentation and organic debris entering the stream. Stream banks that are unprotected by vegetation and roots are more likely to erode, increasing sediment load, which can, if left untreated, lead to changes in riverbed structure and water quality. Overhanging vegetation no longer offers cover against predators for fish and aquatic insects or larvae, nor supplies terrestrial insects and organic matter to the stream.
If left untreated, longer-term effects may include the formation of debris jams from dead trees falling into streams, altering water flow patterns and even creating permanent changes to the stream channel. Nutrient concentrations can remain high for a number of months, possibly as a side-effect of changed nutrient cycling or decomposition of woody debris. Without treatment, sediment movement can take years to recover to pre-fire conditions.
