May 2 Rain Event

May 2 Rain Event
Water Quality - 5/5/2022
We had some interesting weather at the office on Monday (May 2), but not everyone in the watershed experienced the event. At the LOC weather station on our office building roof we received a combination of hail and rain amounting to almost 2.5 inches of precipitation for the day. A bit to our south near Hallinan they received over three inches from the storm. However, it was another story two miles away at the Lake Grove swim park weather station, where less than 3/4 inches fell. This spotty weather was the case throughout the watershed as the storm moved through the area from the NW to SE, with heavy rain occurring in a relatively narrow band.

Rainfall totals for the Lake Oswego area on May 2, 2022. Data comes from Weather Underground (purple circles) and LOC weather stations at the office and Lake Grove swim park (blue circles). Data from Weather Underground has not been validated so some of the information may not be accurate. However, it provides a general sense of the storm path.

Not only did we get a lot of rain that day, but the intensity of rain was very high as well. The LOC offices received 1.07 inches between 4-4:30 Monday, with a maximum rainfall intensity of 8.23 inches an hour. This can be seen in the header image, which is a screen capture of rainfall intensity at 4:05 pm. The red blocks are reas with most intense rainfall with blue blocks the least intense. The lake is not visible on the map, but the line of red blocks are right over the LOC offices. Receiving over two inches of rain in a day is not unheard of and has happened several times over the past winters. The problem comes when rainfall is concentrated in a short time period.

If we receive two inches equally spread out over 24 hours the water slowly runs off impervious surfaces to the lake. Water that hits the ground has a chance to soak in before running off. However, when we get over an inch in 30 minutes the system is not equipped to handle that volume. Storm drains back up, water sheets off the ground instead of soaking in, debris is mobilized and carried downstream, and trash and debris are washed off roads and parking lots into the local stream network. The high volume of water overwhelms streams that have been channelized and disconnected from adjacent wetlands, forcing water over the banks, carrying soil and vegetation with it. All this ends up in the lake as phosphorus rich soil to feed algae and cyanobacteria blooms.

Fortunately this was not a widespread storm, with only a few areas receiving heavy rain. Mountain Park and the Springbrook watershed got a lot of rain, but the Lost Dog watershed and west end did not see as much. As a result we kept up with incoming rain using our hydroelectric generators so the lake level did not dramatically increase and debris flows were isolated to select watersheds.

These intense storms are one effect of global warming. Warmer water evaporates quicker, putting more moisture into the air, which converges into storms inland. The additional moisture has led to an increase in single day heavy precipitation events. We need to make sure our watershed systems are able to accommodate this increase in rainfall intensity, thereby reducing flooding and erosion.

This is why the work of watershed councils is so important. Their focus on healthy soils, native vegetation, and stream restoration will make watersheds more resilient and better equipped to manage large rain events. The Oswego Lake Watershed Council has several work parties throughout the year that are opportunities to learn about soil health, help remove invasive weeds, and plant natives. Maintaining a healthy watershed is vital to keeping lakes and streams safe for wildlife and recreational use. We encourage folks to become involved in the watershed council to learn about and advocate for watershed health.

“Having good healthy soil in our upland areas is an important part of storm water management too. We want plenty of pore space available to hold that water in place. That pore space is stabilized by the aggregate glue produced by a robust population of soil microorganisms.  Our wonderful urban ecosystem working together.” 
     ­­-Stephanie Wagner, President of Oswego Lake Watershed Council