Floods with a Purpose by Tim Hyatt, fall 2003

This article was written by Tim Hyatt, Hydrologist and former SFEG board member, after the 2003 floods on the Skagit River. Its message seems as timely today.

The recent floods on the Skagit River and elsewhere reminded us all that rivers are dynamic systems with harsh penalties for humans and fish that can’t adapt to occasionally extreme conditions. But whereas humans have the option of moving uphill, fish have to either find a refuge somewhere in the river system or else be swept downstream, often to a premature death. While it’s clear from the counts at the Mount Vernon smolt trap that high floods greatly reduce salmon egg-to-fry survival, floods also create and maintain the habitats on which salmon and other fish, plants, and animals depend.

High flows provide ecological benefits by maintaining ecological diversity and productivity. A dynamic patchwork of different habitats—in mainstems, side channels, tributaries, estuaries, sloughs, braids, and wetlands—is necessary for fish to find the habitats they need at different stages of their life cycles. So the mainstem this year may be the side-channel or slough next year, and it is the flood flows like we had last month that trigger these shifts in habitat. Over time the off-channel areas— especially side channels and sloughs–change in character and develop a higher diversity of plants, insects, and habitat types. This higher diversity is often linked with higher productivity as well. After a few years or decades the side channels may once again become the main channel, but in the mean time they have provided some very valuable spawning, rearing, and refuge habitat. Without floods these side channels more quickly become colonized by trees and shrubs, and eventually become blocked or filled or otherwise useless as fish habitat. Maintaining these off-channel habitats, and their connectivity to the mainstem, is crucial for wild salmon to survive.

High flows cause the gravel and fine sediment in the riverbed to move around, in both favorable and damaging ways. Floods inevitably move, re-arrange, and sort the gravel, creating high-quality spawning beds free of fine sediment. Spawning gravels that fill up with fine sediment either suffocate the eggs or trap them in place before the salmon fry have a chance to emerge. Stirring up and sorting the gravel also encourages growth of the aquatic insects upon which salmon and trout feed. Depending on the timing, floods can destroy the eggs that have already been laid, but here too the fish have evolved a mechanism to partially withstand such assaults. Recent studies in the Nooksack basin are showing that egg damage during floods is much less likely in the off-channel areas than in the mainstems, so again the habitat-forming mechanism at work during floods is more important than ever.

High flows, if allowed to function in natural systems, have numerous other ecological benefits. Most of the plant species one finds on river banks are adapted to life in areas with frequent floods, and many of these plants, namely cottonwoods, often can’t get established without barren, wetted areas free from competition— exactly what is left behind after a flood. Floods draw big and small trees and logs into the channel, and it is this large woody debris that, when incorporated into the channel, provides most of the high-quality rearing habitat necessary for young salmon and trout. Floods rejuvenate the floodplain (in both wild and agricultural areas) and encourage the growth of healthy streamside plant communities. Floods clear new areas where noxious weeds can easily take hold, but they also prevent non-native species (both plants and fish), that are not adapted to frequent flooding, from getting established.

Floods have always been with us and always will be. The challenge seems to be to recognize and accept the inevitability of floods and get out of the way. Frequently the most cost-effective means of flood control is to allow the river room to shift and expand. This not only cuts down on revetment maintenance, it also makes for better fish habitat. Trying to limit large floods by increasing dam storage, and trying to limit damage by putting levees close against the low-flow channel, not only deplete tax coffers and destroy fish habitat but often fail as well. Floods like the ones this fall undoubtedly destroy many salmon redds, but they also create and maintain off-channel habitats. So floods may actually be a good thing, if we have the wisdom to let them function naturally.