Atmospheric River and Fertilizer

Atmospheric River and Fertilizer
Water Quality - 12/17/2019

I normally start my updates with discussion about the weather, and this is going to be no different. The weather affects lake water quality in many ways, with warm summers promoting cyanobacteria blooms, to winter storms washing sediment and debris into the lake. This summer was cooler than the past few years and water quality was very nice as a result. When all the lake data are analyzed I will discuss this in a future blog post, but today I am going to discuss more recent weather events.


Air and water temperature and rainfall for June 1 to September 30, 2019, the end of the 2018/2019 water year. The left y-axis is rainfall amount and the right y-axis is air and water temperature in degrees F. The grey shading represents the daily air temperature range with the bottom edge of the shading the daily low and the top edge the daily high. Red circles show the lake temperature near the surface. Blue circles show the lake temperature near the bottom. If only one circle is present that means the surface temperature is the same as the bottom temperature. Columns near the bottom of the graph show daily rainfall amounts. The horizontal blue line marks 32 degrees F, or freezing.

It has been a dry fall so far with only around an inch of rain falling in November. This was preceded by a dry October where we received a little over half what is normal. This has been the case the past three years but this fall is particularly dry and hopefully we start to catch up this month.


Monthly rainfall totals for the last four water years with the start of the 2019/2020 water year shown in green. October and November 2019 rainfall totals are well below normal. Both December 2016 and October 2017 were well above normal rainfall.

In addition to extended dry spells, another effect of climate change is very brief but intense rainfall periods. On December 12 a storm cell moved through the area that produced half an inch of rain in 10 minutes. The most intense rainfall was at 12:47 when 0.11 inch of rain fell in a single minute. Although not sustained, this rate of 6.6 inches an hour quickly overwhelmed the storm drains in the area and caused very localized flooding.


Rainfall the afternoon of December 12, 2019 when the most intense rainfall rate was over six inches per hour.



These intense storms send a lot of muddy water and debris into the lake, providing a source of phosphorus for summer algae blooms. In addition to the trash and leaves shown in the picture there is also animal waste and lawn fertilizer you don’t see. The buildup of soil and rocks where outfalls enter the lake will have to be removed by future dredging in order to maintain navigation and remove the nutrients.

We are expecting more heavy rain Thursday but it has been so dry the rivers are not expected to flood. However, there may be more localized flooding if it rains hard in a short period of time so make sure the storm drains in your area are clear, and have your neighbors do the same. Water will bypass a plugged catch basin, adding to the volume of water running down the street. As we saw last week, it does not take long for the system to become overwhelmed.

Phosphorus-Free Fertilizer
Oswego Lake has had a phosphorus problem for many years. The result has been an abundance of algae, which diminishes water quality. Over the years we have identified many of the phosphorus sources and have been successful in reducing its concentration, and reducing the volume of algae in the lake. This has been a long process started 20 years ago by my predecessor.

Prior to my time at LOC Steve Lundt started a phosphorus free fertilizer program for lake residents. We were educating lake residents about how phosphorus from fertilizers can get into the lake, but they had no ability to purchase a phosphorus free fertilizer. This changed in 2000 when the P-free program was initiated.

There were several steps involved, but an instrumental part of the project was testing soil around the lake to see if phosphorus was needed – and results showed it was not. If there is adequate phosphorus in the soil already there is no reason to add more. The excess phosphorus runs off into the lake and contributes to algae blooms and excessive plant growth. Based on the soil testing the LOC had a local fertilizer company formulate a blend for lake residents. The following paragraph is from a document in support of a city-wide P-free ordinance (which did not pass by the way).

In 2001, the City of Lake Oswego tested soil from 13 parks and found 10 of the 13 had phosphorus levels higher than the 20 ppm adequate for healthy turf, (Cook and McDonald, 2005). In 2000 the Lake Oswego Corporation collected soil from 63 locations around the lake. All seven composite samples from these collections resulted in “very high” phosphorus levels from 33 to 70 ppm.   These local soil tests further provide evidence that our soils can support a healthy lawn without additional phosphorus applications. 

That program lasted for several years, but eventually other fertilizer manufacturers starting producing phosphorus free fertilizer, and it became widely available. However, we have not followed up to see if residents are using P-free fertilizer or done follow-up soil testing to determine if it is needed. We are planning to change that starting in 2020. The goal is to find out how many lake residents fertilize their lawns and what products they use. We will also be doing selective soil testing to measure nutrient levels and see if P-free is still the correct policy. Testing done in 2000 showed phosphorus in the soil was more than adequate to support plant growth and that likely has not changed.

The LOC is working hard to keep a safe and clean lake and have made great progress over the past 20 years. However, it is important that residents around the lake and in the broader watershed use only what is necessary for healthy plant growth and not use excessive fertilizer.