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Dave Rolph is a forest ecologist who has worked for The Nature Conservancy since 1988 in both Washington and Hawaii. Prior to working with the Conservancy, he was a biologist for the U.S. Forest Service’s Pacific Northwest Research Station, where he investigated wildlife and forest habitat relationships in old-growth coniferous forests. Get Involved Join the Conservancy's online community and you can explore new places, receive email you want and build your own personalized nature page! “Roads can completely change the way water moves in a watershed." —Dave Rolph, forest ecologist for the Conservancy in Washington   Enlarge photo Chehalis River overflows its banks, December 2007 Enlarge photo Chehalis River overflows its banks, December 2007 Enlarge photo Chehalis River overflows its banks, December 2007 Go Deeper The Nature Conservancy in Washington Join a member field trip and learn how the Conservancy is saving the last great places in Washington. Global Forest Conservation Why are forests essential for life on Earth? Find out and see what the Conservancy is doing to protect and restore forests around the world. How You Can Help Forests Discover 10 simple ways you can help protect and restore the world's forests.

Record-setting rainfall can wreak havoc, as residents in southwest Washington learned in early December 2007. 

A pounding rainstorm dumped amazing volumes of water over western Washington, driving the Chehalis River over its banks and shutting down Interstate 5 for five days. More than 1,000 homes were damaged or destroyed.  
 
What role can forests play in moderating the effects of such massive storms? To find out, Nature.org talked with David Rolph, a forest ecologist and senior conservation advisor for The Nature Conservancy.

 

Nature.org: How does a healthy forest protect a watershed from flooding?

David Rolph: A healthy forest acts in a number of ways.
First, it intercepts rain. As the rain is falling through the forest canopy, some will land directly on the forest floor, some will land on branches and slowly trickle down to the floor, and some will evaporate off the branches.

In the Pacific Northwest, mosses and lichens in the tree branches are very effective at storing water, and old-growth forests can harbor huge mats of mosses, lichens and soil in the tree canopy — called epiphytes.

When the water does make its way to the ground, an undisturbed forest floor will store water a lot longer, so you have slower runoff. Any forest system is better than impervious surfaces like roads, rooftops and parking lots, which greatly increase the amount and speed of runoff.
 

Nature.org: How can the forest floor store water?

David Rolph: A healthy forest floor in the Pacific Northwest typically has a large organic layer. It can include mosses, downed wood and decaying plant matter.

This organic layer acts like a sponge, absorbing and holding water. The water runs off over time or is absorbed back into the vegetation.
 

Nature.org: What about snow? How do forests and snow interact?

David Rolph:  Because slight changes in temperature or wind can have a big effect on snow, forests can help retain snow longer.

Snow is captured by tree branches and reaches the forest floor in complex patterns. Rather than a solid blanket of snow, you’ll have gaps or depressions under the tree canopy.

The forest understory also has its own microclimate that moderates temperature fluctuations and reduces wind. Warm winds actually melt snow more quickly than rain does.

So snow melts more slowly in a healthy forest than in an opening or clearcut, where it melts more quickly and produces more and faster runoff.
 

Nature.org: What effect do roads through the forest have?

David Rolph: Roads can completely change the way water moves in a watershed. Water will run down a slope and hit a road and then flow along the road ditch to the stream, so the roads are in effect extensions of the stream system.

Stream runoff builds up more quickly in watersheds with extensive roads, so you’ll get higher peak flows over a shorter duration in the stream.

Studies have also shown forest roads can disproportionally act as initiation points for landslides, due to the way runoff moves along roads and the way roads can be loaded with sidecast or spoils from construction.
 

Nature.org: Some of the most spectacular effects from the December flooding in Washington were big mudslides. What role do forests play in preventing landslides?

David Rolph: There were rumors of as much as 20 inches of rain in 24 hours down around Chehalis. When something like that happens, you are going to get landslides, no matter what.

But a healthy forest will have greater root strength to resist slides, in addition to moderating the effects of runoff. And a landslide in a healthy forest actually delivers important wood structures to a stream — which increases habitat complexity — while a landslide in a clearcut brings mostly sediment to the stream.

In the last decade, Washington adopted new forest practice rules that will greatly improve forest road systems and lessen the impacts from forest harvesting during heavy rainfall events.

But we still have to see over time how far improved management goes towards abating the continued threat of flooding and landslides.

Nature picture credits (left to right, top to bottom): © Vern Potts/WSDOT (Chehalis River flooding); © Dave Rolph/TNC (Dave Rolph); © Vern Potts/WSDOT (Chehalis River flooding); © Vern Potts/WSDOT (Chehalis River flooding); © Vern Potts/WSDOT (Chehalis River flooding)