Chehalis River, a Bit Different Than Other Western Washington Rivers

I have been following the water levels on various western Washington rivers during this string of warm rainy days. Lots of moderate flooding throughout western Washington rivers with the Snoqualmie/Snohomish system flooding the most. Yesterday it cooled down and thus the flooding was reduced as mountain rain changed over to snow. NOAA has a great site with forecast river levels for northwest rivers at http://www.nwrfc.noaa.gov/.

In checking the weather forecast I came across the following on the NOAA hydrology forecast discussion:
HYDROLOGY...FLOOD CRESTS ARE MOVING DOWNSTREAM INTO THE LOWER REACHES OF RIVERS THROUGHOUT WESTERN WASHINGTON THIS EVENING...WHILE THE MID AND UPPER REACHES ARE RECEDING. THE LOWER REACHES OF MOST RIVERS WILL CREST LATER TONIGHT. THE EXCEPTION IS THE LOWER REACH OF THE CHEHALIS RIVER IN GRAYS HARBOR COUNTY. AMONG THE RIVERS IN WESTERN WASHINGTON...THE CHEHALIS IS UNIQUELY LONG AND SLOW MOVING. THE CHEHALIS AT PORTER WILL CREST EARLY TUESDAY MORNING...AND IT WILL TAKE ANOTHER 24 HOURS FOR THE CREST TO MOVE DOWNSTREAM TO GRAYS HARBOR.

The Chehalis River, located in southwest Washington, really is very different compared to other western Washington rivers.

Washington Rivers, USGS

As the NOAA site points out the Chehalis is "uniquely long and slow moving". I'm not sure the length is particularly unique, but the Chehalis system certainly is unique relative to other rivers in western Washington of comparable size. So I came up with a set of unique features for the Chehalis relative to other western Washington rivers that likely impacts the flood hydrograph of the river:

1) The Chehalis drains the western eastern side of the Willapa Hills, the Black Hills, an area of low mountains on the west side of the Cascade Range, and the lower south slopes of the Olympic Range. Hence this river gathers water from four ranges. Other western Washington rivers are limited to one range only. I would note that even though the Willapa Hills and Black Hills are called hills that within the Willapa Hills and Black Hills there are several peaks that are called mountains.

2) Other western Washington rivers have headwater areas at much higher elevations. The headwaters of the Chehalis are much lower. All of the other rivers have headwaters that include glacial areas. Hence, most of the other western Washington rivers flooding subsides rapidly as storm systems cool and switch over to snow such as what happened yesterday. 

3) The Chehalis is also the only river that drains water directly from the Cascade Range to the ocean. The other Cascade drainages drain to Puget Sound or the Columbia River. 

4) Because the Chehalis starts at lower elevation and has to flow all the way to the Pacific, it has a much lower gradient than the other rivers. Hence, it takes longer for water to move through the system. It is as NOAA points out a slow moving river. Besides the elevation and length, there are a couple of other factors effecting the gradient. One is the lower Chehalis occupies a valley that it did not carve. The Chehalis follows the former valley of a much larger river. During the maximum ice extent during the last glacial period melt water from the Puget lobe ice sheet drained to the ocean via what is now the Chehalis River. The river that carved that valley was a much bigger river than the Chehalis. This glacial outwash river flowed into the ocean at a significantly lower elevation and significantly west of the present mouth of the Chehalis. During the ice age sea level was a lot lower. As sea levels rose after the last glacial period,  the lower end of the Chehalis River flowing down the old glacial river valley was flooded forming Grays Harbor and backed up by the rising sea  further slowing down the Chehalis.

5) A fifth factor that makes the Chehalis different is that it drains an area that was for the most part never glaciated. Soils in the Chehalis are deeper and respond somewhat differently to water inputs from big storms.

The Chehalis has a had a long history of flooding with a few floods the past couple years that has generated significant policy debate regarding land use zoning, flood control, forestry, transportation (Interstate-5 was closed), and global warming. I am sure the Chehalis will make Washington State news again, and part of its notoriety is that it really is different than other western Washington rivers.