Reservoirs are parched and ready for winter

Eye on Climate
The Bull Run reservoir. (Portland Tribune)_463100

PORTLAND, Ore. (KOIN) – We have made it to the halfway point of fall, which means one step closer to the winter snows. This is when we bring in opportunities for lucrative atmospheric rivers.

We have already had a handful of both, which is a positive sign for early November. Even with the rain coming in and out, for October and November, we have to keep this all going for winter and spring. It’s a ride that we make a pact with because we know how important it is for our water cycle come summer.

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File. Detroit Lake Dam

According to hydrologist Andy Bryant, with the National Weather Service in Portland, the reservoirs in the northwest essentially get a restart each fall. The winter snow and spring rain help fill us up, and Bryant says, ” [the water] is released in the summer to maintain water quality, including cooling the river temperature downstream of reservoirs, and supplying water for several irrigation districts and municipal users in the Willamette Valley.” We know how important this process is, especially with ups and downs from year-to-year.

We can currently say that the northwest is managing near normal for streamflow compared to historical streamflow for this day of the year. The map provided below from the U.S. Geological Survey (USGS) has the Willamette Valley region in the normal percentile class of 25-75. Much larger issues for central Oregon and down through the Klamath region. It has been better in the northwest because of the recent rainfall, which has been above average going back to September. That late surge of September rain helped boost our water year from what could’ve been one of the top 5 driest on record. The question now, is will we be able to bring in the necessary water for the dry sectors of the Columbia Basin? The Cascades produce a rain shadow over the central core of the state, which is now in urgent need for water.

River basins are going to appear grim right now. As noted above, this time of the year is when we have a restart. In other words the reservoirs start low in preparation for what’s to come. Some of the larger, notable storages, may be anywhere from near empty to 10 to 20 percent. Scoggins Dam and Henry Hagg lake are currently sitting at 28 percent full. Which is higher than many of the reservoirs that you can see in the graphic below. It is possible that some of the reservoirs are low because of the summer conditions and the operations that were necessary at that point of time. With exceptional drought conditions impacting the state, water has been hard to come by (impacting soil moisture).

A shaving of the northwest Oregon coast is free (1.34%) of the drought conditions. It’s not saying much, but it’s a trend in the right direction. We are still working to improve our drought conditions in the Willamette Valley. There is a sharp difference over the span of 40 miles. Drought conditions may be hovering around dry or moderate, with extreme drought conditions just 40 miles away in the southern Willamette Valley.

It is typical that the northwest corridor of the state receives more rain than the neighboring sections. Astoria usually takes the crown when it comes to rain totals (16.63 inches since September 1). It has been the rain going back to September that has helped that section tremendously. Henry Hagg lake is much closer to that massive improvement, where as the other reservoirs may be stuck in the shadow of the mountains.

Let’s focus our attention more on the Willamette Basin, which is mainly controlled by the US Army Corps of Engineers. We will not focus on the numbers, but you can see how the reservoirs are all currently settled near the bottom. The total for the Willamette Basin will reflect that, but the nature of the situation isn’t as devastating as it would be in the summer. However, drought conditions are still prevalent from the north, to the south. The Willamette Basin, which keeps a massive load of population running, is looking forward to the incoming rain and winter season.

Courtesy: https://www.nwd.usace.army.mil/

Lastly, I have two graphics here of the monthly water supply from The Dalles Dam and the Bonneville Dam. You can see where the 2021 volume compares to the 30-year normal and the wettest and driest 5 years. It is clear that it is much lower this year compared to the 1981-2010 normal. The Bonneville Dam is just about to the 30-year normal as of October, which should represent an increase here because of the productive fall months. To put this all into perspective, you can see which months are the most important. The surge from spring into the summer is when we reap the benefits of the wet season.

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