What a flood does

December 30, 2008 11:00 pm
The water intake on the north bank reflects in flood water Monday morning. (The Pilot/Chad Robert Snyder).
The water intake on the north bank reflects in flood water Monday morning. (The Pilot/Chad Robert Snyder).

By Chad Robert Snyder

Pilot staff writer

Most of the impact of high water is obvious. It's hard to miss flooded roads, debris-littered torrents or eroded hillsides.

What might not be so easy to see, though, are some of the most significant impacts a flood can have.

To begin with, in an attempt to accurately gauge the result of the recent flooding of the Chetco River system, it's important to know the relative severity of the Dec. 27 to 29 event, which peaked at a flow of nearly 51,000 cubic feet per second (cfs)

To put it in historical context, the United States Geological Survey (USGS) refers to a 50,000 cfs flow in the Chetco as a "20 percent probability for any given year" event, meaning each year there's only a one five chance of such conditions occurring.

According to the USGS, there have been only six events over the 50,000 mark since 1970 when they began accurately measuring the Chetco flow with a new measuring station. Three of those have occurred since 1996.

What this translates to is a sizable impact on the Chetco, but one, according to Oregon Department of Fish and Wildlife Biologist Todd Confer, that has positive effects that somewhat offset the negatives.

"What you have in these large flows are potentially channel changing events," he said. "It returns the system to a less mature state."

Confer clarified the statement by saying that intense flows change a number of river features, including riverside vegetation and the levels and locations of gravel bars.

This, in itself, has an upside and a downside: Erosive water behavior can negatively effect habitats, but it can also "churn up" gravel, making it more "user friendly" for breeding salmon and steelhead.

The destructive force of the water can also change the "channel morphology," sometimes for the better in terms of fish and fisherman.

Confer said the creation of new gravel bars can create larger and wider pools, which give fish resting places and anglers a ready idea of where fish may be hiding.

From Confer's perspective, the result to this year's event – despite dislodging eggs and increasing the mortality rate for salmon fry – will ultimately help the native fish population.

"It will certainly make changes to the habitat that will be beneficial for Chinook and steelhead in the next few years," he said.

In the steelhead's case, a less "mature" river will improve their access to tributaries, where, as Confer said, steelhead prefer to spawn.

In general, native fish populations are accustomed to changes in river systems.

"Fish are adapted for this stuff," Confer said.

Despite the temporal nature of many of the effects of flooding, some can have longer-term impacts.

"Traditionally, geomorphologists refer to a two-year (50 percent probability for any given year) flood event as a channel forming discharge," said USGS Hydrologist Rose Wallick. "Clearly the larger flows (like the recent Chetco event) have a more dramatic affect on the river system, which can last years to centuries to tens of thousands of years."

This is of particular interest to the USGS, which is conducting an ongoing "Sediment Transport Study."

The goal of the study is to accurately gauge the "sediment budget," which monitors the average annual amount of gravel and debris entering and exiting the Chetco system.

As Wallick said, a flood of this caliber affects the "budget" and changes the shape and character of a river in unpredictable ways.

"In some places there are depositions, in others, erosion," Mallick said.