The cold and dry weather this month has not helped our refill efforts as we have received a bit over a half an inch of rain since starting the refill December 1. It looks like it is going to be dry until at least the middle of the month and we are not likely to get our average rainfall amount of around 5.5”. For the past week we have only gained about a half an inch of elevation per day; good news for contractors on the lake, but gives us pause about how soon the lake will fill.
The last time we relied on rainfall exclusively to refill the lake was in 2007, and it took three months and almost 16 inches of rain to recover from the 10-foot drawdown. Our subsequent drawdowns in 2010/11 and 2014/15 required Tualatin river water to complete the refill. The LOIS recovery in 2011 required river water because of the very deep drawdown and the late refill. The 2015 drawdown was started earlier but we only received 12 inches of rain from January to April, again requiring river water to top the lake off. This is the reason we did such an early drawdown this year, refilling December 1 instead of January 1. The way things are going it may take a few months to reach 15 inches rain so we need to start as soon as possible.
One thing I like to do during our drawdowns is look at the outfalls and see how much sediment has been deposited. Lots of areas of the lake receive sediment during winter, but three in particular collect significant debris. These three locations were dredged during the 2010/11 LOIS drawdown so it is informative to see how quickly sediment is replaced after it has been removed.
Lost Dog creek has a very steep drainage that moves a lot of material to the lake during high flow runoff. We removed around three feet of material from this delta during the 2011 LOIS drawdown, but six years later the mouth of the creek is filled with debris again. Actually I wrote about this
You can see from this picture the size of material that comes down the creek. During heavy rain the energy from water rushing down the creek can mobilize football sized rocks. As these large rocks tumble down they strike and dislodge other rocks and soil, further contributing to erosion. Large material settle out near the mouth of the creek while organic material and silt continue further into the lake.
Sediment at the Lost Dog Creek delta that has accumulated since our 2011 dredge.
Lost Dog Creek drainage basin
A large storm outlet where the South Shore bridge crosses Blue Heron canal also deposits noticeable sediment. We dredged this area in 2011 also and a lot of sediment has filled this area. It is difficult to see in the picture, but there is about a foot of material to the left of the red line that originated from the outfall.
Fan of sediment in Blue Heron Canal near the Southshore bridge
Drainage basin for the Blue Heron outfall near Southshore bridge
Sediment deposited in the Springbrook delta leaves a seasonal pattern of organic debris separated by silt. During fall leaves and twigs fall into the creek and are transported to the lake. In winter the heavy rains bring sediment that covers the leaves. The next fall the process repeats and over the years you get a layer cake effect where leaves and tree debris alternates with sediment. During a drawdown the creek cuts a channel through these layers, exposing the strata. Currently there are four distinct layers of leaf and sediment in the creek delta.
Different layers of sediment representing seasonal deposition in the Springbrook Creek delta
Watershed for Springbrook Creek
All three of these locations have in common a very steep watershed and lots of impervious surfaces that shunt large volumes of water towards the lake. Despite our dredge activities these areas continue to fill, and will do so until more water from impervious surfaces is infiltrated into the ground instead of being piped to the nearest stream. Unfortunately, LOC has no control over what happens in the watershed, but we are leading by example with our building remodel by including flow-through planters that clean runoff before it enters the lake.
Our storm system and most homes on the waterfront are piped directly to the lake so no stream erosion is caused by runoff. However, cleanliness of water is very important. In a flow-through planter the rainwater is piped from the roof to the planters where it infiltrates through about two feet of soil before exiting via a drain at the bottom. The planters contain vegetation that takes water and nutrients from the soil as water passes through. This process removes leaves, pine needles and pollutants before it gets to the lake. During small rain events there may be no water that exits the planter because vegetation takes up the moisture or it evaporates prior to the next storm.
Sediment deposition is a natural process for any lake, but impervious surfaces in Oswego Lake’s watershed stemming from intense development in a steep watershed has greatly accelerated the process. In addition, soil in the watershed is highly variable, with some very porous and able to accept rainwater and others clayey that very slowly accepts groundwater. The result is an uneven ability to infiltrate runoff from roofs, streets and parking lots. Compound this with the historical practice of piping all runoff to the nearest stream and you end up with a watershed level excavation project during rain storms that moves tons of material a year into the lake.
Our algae season is over and it looks like we have entered winter weather conditions. This means the lake level will be fluctuating as rain comes in and we run our hydro generators.
The LOC intern has been collecting data on Oswego Lake this summer and will continue through next year. She recently deployed sampling buoys on the south shoreline to collect data for her research.
The weather in July was hot but the cyanobacteria did not get out of control.
As of this writing the lake level is 97.6 feet above sea level, meaning there is still just under a foot to go until full.