Dear Lady of the Lake,

We sure have been getting a lot of rain, we need it. My question is: How much rain will it take to fill Clear Lake to full, like a normal year? How do I find out where to see the lake levels and when do we have to worry if it will flood?

Thanks!

From Raining on Randy on Rocky Point


Dear Raining on Randy,

Thanks for this excellent question! I have received this question from several folks, so I thought it best to tackle it in today’s column.

First I will provide some important links and resources for learning about lake levels and stream heights and flows. Then I will provide you with some calculated outlooks for where the lake is going with the rain we received and what we can expect to see with any more rain. I will also provide you the estimated amount of rain needed to get a “full” lake, and the amount of rain that would lead to several different flood level lake stages.

Measuring Lake Levels

Clear Lake level is determined in a very unique way, using the Rumsey gauge to measure lake levels. I described this in my column, “Learning about Lake Levels” from Oct. 17, 2021.

Daily lake level data is measured from an in-lake gauge (#11450000) operated and maintained by the United States Geological Survey (USGS). The gauge has historically been located on a pier at Lake County Vector Control District in Lakeport, but sometimes during droughts, when there is a lack of water depth at the gauge’s normal location, the gauge will be temporarily moved to the 5th street ramp at Library Park in Lakeport. USGS maintains that all online data is provisional until verified and confirmed and usually that can take several months.

The lake level data is available online in graph or tabulated format at the USGS Clearlake Lakeport CA station.

The lake, when full, will be 7.56 on the Rumsey gauge. As I write this article, the lake is currently at 2.84 Rumsey, so about 40% full. But remember, that we were below zero Rumsey in December, at about -2.25 Rumsey, until we started getting these rain events, so we had to get to zero first, before rain could start to fill the Lake to full.

However, if you notice the historic median lake level on the graph (dotted, top line), you will notice that we have met the historic median, making the lake level today very similar to the previous 109 years, with 50% of the years being more than that value and 50% being less (definition of median).

If you want to know stage height data from other gauges around the state, I suggest you explore the Sierra Nevada River Forcast Network Interactive Map. On this site, you can find different lakes or rivers with real-time gauges measuring lake levels, stream height and stream flow.

This also has a great interactive map, where you can see what gauges are where and what type of information they are collecting such as precipitation, stage height, flow, and some gauges also provide forecast conditions.

Also, the Lake County Water Resources Department monitors the daily lake level and can provide that to you by phone at 707-263-2344, if you do not have access to the website.

Streaming real-time stream data

There are lots of resources available online for anyone to access real-time stream and river data. Please refer to my previous column, “Lady of the Lake: Streaming stream and river data in real-time” from October 31, 2021. 

In that column, I reviewed two of my favorite websites for accessing stream data, including The California Nevada River Forecast Center, the CNRFC, and California Data Exchange Network, or CDEC.

The CNRFC, is a field office of the National Weather Service located in Sacramento, California. The CNRFC is co-located with the Sacramento NWS Forecast Office, CDWR, and United States Bureau of Reclamation. The NWS is an agency of the National Oceanic Atmospheric and Administration, or NOAA, under the United States Department of Commerce.

The CNRFC is great for monitoring flash flooding, river forecasting, managing water resources, or learning about hydrometeorology. Also the maps and graphs are very easy to use and interpret. This site can also provide temperature conditions and predictions which can be used for frost protection planning, for example. Additionally, this site has incorporated some post-fire debris flow warning layers. So if you live in an area that has a burn scar nearby, this website can help you to visualize those areas most at risk to post-fire flooding and debris flows.

The next resource is California Data Exchange Center, or CDEC. This website is managed by the California Department of Water Resources and while it does have some prediction power, the majority of gauge information here is real-time and archival. However, it’s pretty easy to use and has a large network of gauges and data that expands across all of California.

One thing about the CDEC is the data is focused on river information. The tabs at the top of the home page provide a lot of the type of data and tools available, and makes it easy to search for what you might need. Historic water data is also easily available on CDEC, making it easy to conduct a study on your local stream or river.

One thing to be aware of with CDEC is that the website can get pretty busy and bogged down during popular use times. If there is a heavy storm event and people and agencies are looking for river data and flood conditions, this website might get really slow and pages might not load as quickly, so be patient.

In my previous column on stream data, I provided a video tutorial on how to access CNRFC data from your own computer. This video is very useful for someone who has no prior experience getting stream data from online resources.

Stream and Lake gauges in our area

Here I provide some of the quick links for the local gauges for streams and lakes of interest.

Scotts Creek at Eickhoff Rd. Gauge California Nevada River Forecast Network
https://www.cnrfc.noaa.gov/graphicalRVF.php?id=SKPC1

Middle Creek at Rancheria Rd. Gauge California Nevada River Forecast Network
https://www.cnrfc.noaa.gov/graphicalRVF.php?id=MUPC1

Kelsey Creek at Kelsey Creek Dr. Gauge California Nevada River Forecast Network (Kelsey Creek Upstream of Kelseyville)
https://www.cnrfc.noaa.gov/graphicalRVF.php?id=KCVC1

Kelsey Creek gauge CDEC (Downstream at Soda Bay Road)
https://cdec.water.ca.gov/jspplot/jspPlotServlet.jsp?sensor_no=20400&end=&geom=small&interval=2&cookies=cdec01

Cache Creek at Rumsey Gauge California Nevada River Forecast Network (Yolo County, CA)
https://www.cnrfc.noaa.gov/graphicalRVF.php?id=RMSC1

Putah Creek t Guenoc California Nevada River Forecast Network
https://www.cnrfc.noaa.gov/graphicalRVF.php?id=PCGC1

CDEC for Clear Lake and Cache Creek System (aggregate of 9 River and Lake sites)
https://cdec.water.ca.gov/river/cacheStages.html

Below is a list of some *NEW* stream gauges installed this year thanks to the Blue Ribbon Committee for the Rehabilitation of Clear Lake Funds with partnerships from USGS, UC Davis, County of Lake Water Resources, Habematolel Pomo of Upper Lake, and City of Clearlake. These gagues are temporary but when fully operational will report stage height, flow, water temperature, and turbidity (how cloudy the water is).

Cole Creek in Kelseyville, CA (USGS)
https://waterdata.usgs.gov/monitoring-location/11449820/#parameterCode=00065&period=P7D

Molesworth Creek at Old 53, City of Clearlake (USGS)
https://waterdata.usgs.gov/monitoring-location/11449370/#parameterCode=00065&period=P7D

Clover Creek, Bypass channel at Elk Mountain Rd. in Upper Lake, CA (USGS)
https://waterdata.usgs.gov/monitoring-location/11449235/#parameterCode=00065&period=P7D

Scotts Creek at 29 Bridge in North Lakeport, CA (USGS)
https://waterdata.usgs.gov/monitoring-location/11449255/#parameterCode=00065&period=P7D

How much rain will raise Clear Lake?

Now, to answer your last question Randy; how much rain does it take to raise Clear Lake levels and how much is too much rain, or will result in flooding conditions.

It’s kinda remarkable that a month ago we were worried about another year of severe drought and now we are talking about flooding, but that is the new climate paradigm we are living in.

If you read my previous column about streaming stream data, you will notice that I refer to myself as a “data snob” because I like data clean, organized, and I like to use data to answer interesting questions. Randy, your question is very interesting to me, and I researched what was out there when it came to how much rain corresponds to a rise in lake level.

I got several answers, in conversations with folks who have lived here on the lake a long time. The most common answer was that every 4-5 inches of rain, we see about a foot of lake level increase. So, if we got 10 inches of rain, we would expect to see about 2 feet of lake level increase.

Logistically, the lake is rising from the rain itself (collecting in the lake) and the collection of runoff from the landscape and tributaries flowing into the lake contributes to the lake level increase.

Me, being a data snob, needed to see the numbers. Unfortunately for me, I am not an engineer and don’t have an engineering brain, but I dusted off my old mental file of math and algebra and attempted to understand the relationship between the rain and lake level rise. Full acknowledgement to Mr. Blackwell, my high school Algebra teacher - you were right, sometimes we do use this stuff in real life, Mr. B!

First I downloaded daily rain data from CDEC for Lake County, CA. The closest station with daily accumulated rain was the Whispering Pines (WSP, CA Dept of Water Resources/DFM-Hydro-SMN). For these purposes I used accumulated rain, and I converted the data to start from zero in December, as opposed to 10 inches accumulated for the entire water year which runs from October to September. I chose December because that is when we first started getting rain for this series of rain “events” and started seeing significant lake level rise.

I didn’t want to include accumulated rain from earlier in the year, for example from mild showers in October, to interfere with my calculations today, as that rain is probably, most likely, not contributing to lake levels we are seeing now.

Then I downloaded daily lake level data, from the Lakeport gauge station housed at Vector Control, as described above in the beginning of this column. Then I plotted both of these data on a scatter plot, with rain on the X axis (independent variable) and lake level rise on the Y axis (dependent variable). Then I calculated the slope of the data and fit a line to the slope.

Heads up, algebra alert, to calculate the slope of a line, or the “rise over run” of a line, we have to use the formula m=(y2-y1)/(x2-x1), with m= slope, x1 and x2 being two numbers in a list of x (rain) and ya and y2 being two numbers in the list of y (lake level). This is also described as the change in the y values over the change in the x values. The slope will answer the question; how much change in lake level will we see when we have one inch of rain?

A key piece to remember is that up until Zero Rumsey water is only going into the system, there is no outflow from the lake until the lake level gets to Zero Rumsey and only after that can water start to flow over the Grigsby Riffle in Cache Creek.

So this added another variable into my calculations and plot, even though we are still getting quite a bit of rain, we also are seeing water leave the system. So, here we go algebra again, I calculated a separate slope for when the lake is over zero Rumsey, to account for the precipitation, inflow from tributaries, and outflow into and down Cache Creek.

To my surprise (but not really) the slopes are pretty different. The slope for the rain-lake level relationship before zero Rumsey is 0.14 and the slope for after the lake is above zero Rumsey is 0.26. This means that for every one inch of rain, the lake rises about 0.14 feet or 0.26 feet, depending on if there is outflow or not. So it does take roughly 4 inches of rain to raise the lake one foot - when the lake is at or above zero Rumsey.

It takes quite a bit more rain (about 7 inches) to raise the lake one foot when the lake is negative Rumsey. Mathematically this might not make sense and you might be asking yourself “if water is leaving the system down Cache Creek, how can it take less rain to raise the lake?” And I would answer by asking you to consider the system as a part of the watershed in a post-drought year (of which my data is based).

It takes a lot of rain to infiltrate and soak into the soils, and then it takes a lot more to accumulate into runoff that flows into the lake. Another thing to consider is that the outflow to Cache Creek is very limited, at about 2,500 cubic feet per second (CFS), compared to a single inflowing tributary such as Middle Creek, which can easily reach 3,000 - 5,000 CFS in any given large storm event.

Lasty, Randy, you asked when we should see a “full lake”. The lake is considered full when it reaches 7.56 on the Rumsey gauge, and I estimated that if it receives 45 inches of rain (based on the slope for positive Rumsey) we should be at full lake status. This is about 55 annual accumulation from October.

We have quite a bit to go as we have received about 25 inches in the last three weeks and we are only 40% full. You also asked about flood stage, and I have been receiving lots of inquiries about that too.

How much rain does it take to raise Clear Lake? It depends on if the lake is below or above zero Rumsey. Red data points and slope equation is based on rain and lake level data from Dec 23, 2022 to January 8, 2023. Black data points and slope equation derived from rain and lake level data from Jan. 9 to 13, 2023. Rain data sourced from DWR CDEC WSP Station and Clear Lake level data from USGS Lakeport station 11450000.

According to USGS, and based on 109 years of historic lake levels, when the lake gets to 9 ft Rumsey, it's considered minor flood stage, 11.4 ft is moderate flood stage and major flood stage is when the lake is at or exceeds 12 ft Rumsey. According to the Lake County Water Resources Department Historical Levels of Clear Lake High and Low, there is 10% chance of getting to minor flood stage, and 1% chance of the lake getting to major flood stage in any given year.

Using my rudimentary slope calculations, I have calculated how much rain (continuous, mind you) we would need to get to those stages. This is estimated and only applies with the continuation of the rain series we are seeing. If we see a dry month, the trend might be slightly off, but these are generally good guidelines as the ground is currently saturated and the lake is likely to stay above zero Rumsey.

It would take about 40 inches during this rain event to get to the minor flood stage of 9.0 feet Rumsey (50 annual accumulation from October) and about 52 inches (63 annual) to get to the major flood stage of 12.0 feet Rumsey. Notice that my calculated numbers are not based on annual accumulation (starting in October), but total for the rain events starting in December, as the annual year reset on January 1st.

That is quite a bit to go, but anything can happen in a third year of La Niña, as we have learned.

Sincerely,

Lady of the Lake

Angela De Palma-Dow is a limnologist (limnology = study of fresh inland waters) who lives and works in Lake County. Born in Northern California, she has a Master of Science from Michigan State University. She is a Certified Lake Manager from the North American Lake Management Society, or NALMS, and she is the current president/chair of the California chapter of the Society for Freshwater Science. She can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..