Tenmile Creek Hydrologic Outlook
Watch the signs: Groundwater | Streamflow | Snowpack | Weather Forecast
Updated: February 21, 2025
As we begin to see the first signs of warmer temperatures, and we get closer to flood season; the Water Quality Protection District is keeping an eye on conditions.
If you have questions or concerns about flooding, please call our office at (406) 457 - 8584 or email us at JMcBroom@LCCountyMT.gov.
Tenmile Creek
As the Tenmile Creek flows through the central Helena Valley it passes commercial businesses, residential areas, and farm land.
Tenmile Creek (blue line) originates southwest of Helena near the town of Rimini, Montana along the Continental Divide. As it crosses the Helena Valley the stream flows past the Helena Valley Irrigation District (HVID) canals (pink lines on map). Eventually, the stream flows into Lake Helena and on to the Missouri River.
Tenmile Creek Basin
Every body of water has a drainage basin. A drainage basin is the area of land where any precipitation (rain or snow) that falls within this area drains to that body of water.
Tenmile Creek has a drainage basin of 121,000 acres (190 square miles). The map to the right shows Tenmile Creek's drainage basin in teal. All of the rain and snow that falls within this teal area ends up in Tenmile Creek.
190 square miles is a lot of land! How much water flows through Tenmile Creek?
Why does it flood?
Let's say we have a storm go through our area and we receive a half-inch of rain. On top of the normal amount of water in the stream that is always flowing (this is called baseflow), the stream would receive about 5,000 acre-feet of water from the storm. One acre-foot of water is an acre of land covered in water 1 foot deep.
To help give some perspective, here are some other ways to picture this volume of water.
5,000 acre-feet of water is equal to 219 million cubic-feet of water or 1.64 billion gallons of water.
A football field, such as the one at the University of Montana, has an area of 6,396 square yards if you include the endzones (120 yards long x 53.3 yards wide). If we put 1.46 billion gallons of water on top of the football field, it would be 3,800 feet high. This is almost three-quarters of a mile high!
Image Source: University of Montana (https://map.umt.edu/?id=1906#!m/553979?s/)
A full-size pickup truck's 8-foot bed has a volume of around 75 cubic feet. If you tried to haul 1.64 billion gallons water, you would need 3 million trucks (2,961,390 trucks to be exact).
And that is only when a half-inch of rain falls... Imagine if we receive one or two inches of rain!
What do you think happens when we include the spring snowmelt alongside the rain?
With the enormous amount of water being drained through this stream, flooding is unfortunately common.
30-Day Trends
Now that we have looked at why flooding is such as issue along Tenmile Creek, let's look at the recent trends. There are two trends we review when thinking about the potential for flooding: streamflow and groundwater.
Streamflow
We monitor how much water is flowing through a stream by measuring the gage height (height of water in the stream) and the discharge. Discharge is the volume of water (cubic feet) passing a point in the stream per time (second). This is measured in cubic feet per second (CFS). When more water is flowing down the river it has to go someplace. If the stream channel cannot take all of the water, flooding will occur.
In addition to discharge measurements conducted by us, we also use data collected by the United States Geological Survey (USGS). The USGS maintains streamflow-monitoring stations (known as gaging stations) throughout the nation, and all of their data are free to view by everyone. One of their stations is on Tenmile Creek near Williams Avenue. The graph below shows the discharge trend over the past 7 days. Spring flooding of neighborhoods in the Tenmile Creek flood zone can occur when streamflow approaches 500 to 600 CFS at the Williams Ave USGS gaging station.
Click here to see historic data from Tenmile Creek, including the flood event in 2018.
Groundwater
We routinely measure groundwater levels across the Tenmile Creek drainage basin. As groundwater levels approach the surface, the chance of flooding increase. Groundwater even near the surface can flood crawlspaces and basements.
Additionally, as rain and snowmelt attempt to soak into the ground they must have room to do so. If the ground is already full of water, there is no where for the rain and snow to go - this increases the chances of flooding. Imagine a towel that is damp, but not dripping wet. If you go to wipe up a spilled drink, the towel still has the ability to absorb more liquid. However, if that towel is dripping wet, how much of that drink is it likely to absorb?
Streamflow Trends
30-Day Trend as of May 23, 2024:
Towards the end of April the discharge warming air temperatures provided additional water to the stream from snowmelt. The stream was maintaining between 40-60 CFS of flow. Beginning around May 10 the storms passing through our area introduced more water to the stream which increased the discharge to approximately 80 CFS. The most recent rain during the week of May 20th increased the discharge to over 100 CFS.
Spring flooding of neighborhoods in the Tenmile Creek flood zone can occur when streamflow approaches 500 to 600 CFS at the Williams Ave USGS gaging station.
The past 30 days of discharge has been well below flooding levels.
Groundwater Trends
30-Day Trend as of May 23, 2024:
The groundwater levels are shown on the map. The darker-red triangles represent higher groundwater that is closer to the surface and more prone to flooding. Yellow triangles show groundwater levels are lower.
The current groundwater levels near Green Meadow Drive are between 3 to 6 feet below ground surface. However, the groundwater between Mill Road and Forestvale Road continues to stay between 16 and 30 feet below ground surface.
Hint: If you accidently close the map legend, clicking on the round button in the bottom-left corner of the map it will bring it back.
Current Conditions
The WQPD monitors the stream up to 8 sites along Tenmile Creek. These sites provide opportunities to document the stream conditions including measuring discharge, collecting water quality samples, and simple taking pictures of the stream.
We also monitor groundwater levels at numerous wells in the valley that are impacted by flood waters from Tenmile Creek.
The current conditions of Tenmile Creek and the the surrounding groundwater are summarized in the map below. If you are interested in viewing more information about stream conditions or the groundwater, click on any of the circles (stream sites) or the yellow triangles (wells). The information in this map will be updated regularly to help keep everyone informed!
Flood Hazard Layer
In the interactive map below you will see two types of red areas: (1) solid red and (2) striped red. These two areas represent two different flood hazard zones and are based on Flood Insurance Rate Maps (FIRMs) zones designated by the Federal Emergency Management Agency (FEMA). For Lewis and Clark County's FIRM's, please visit our Floodplain Program .
High Risk Flood Hazard Areas (Zones A, AE, and AO)
These three zones along with zones AH, AR, A99, V and VE represent the Special Flood Hazard Area (SFHA). These areas have a 1% annual chance of flooding (100-year flood), which is also known as the base flood. The individual zones within the SFHA area based on the extent of flooding and any flood protection infrastructure.
Non-Regulatory 500-Year Floodplain (0.2% Annual Chance of Flood Hazard)
These areas have a 0.2% annual chance to flood with an average depth of less than 1 foot of water or with drainage areas less that 1 square mile.
If you have questions about that flood hazard zones here in Lewis and Clark County, please contact the Lewis and Clark County Floodplain Manager's office at (406) 447-8375.
Tenmile Creek Hydrologic Outlook
Flood Forecast
We have now looked at the trends from the past 30-days to see what trends have been and the current conditions. Now we must look at what the future holds in terms of snowpack and weather.
As our snowpack melts, it adds additional water to our streams, including Tenmile Creek. Warmer weather accelerates this melting process which can inject more water into our streams within a shorter window of time. Additionally, spring rains directly add more water to our streams in a matter of hours. Only so much water can travel down Tenmile Creek at any given time before flooding occurs.
Remember how much water goes Tenmile Creek with a simple half-inch rain event?
Snowpack
The amount of water held in snowpack (snow water equivalent) in the mountains, can be an indicator of the volume of runoff expected in the spring. The Frohner Meadows SNOTEL site is located in the mountains to the south of Helena and is used as a general measure of the water content of mountain snowpack around Helena.
The BLACK LINE is the current 2024 season's snowpack. Click on any year at the right to show that year's snowpack.
Weather Forecast
Be sure to check the weather for the upcoming week. If the stream channel is already full and more precipitation is in the forecast, flooding is likely to occur!
Flood Forecast
The National Weather Service out of Great Falls has used the USGS stream data to track the risk of flooding for stream gaging locations. This tool shows the relationship between the monitored stream level height (gage height) and the flooding status.
The screen to the right summarizes the current flooding risk for Tenmile Creek.
Interested in other streams besides Tenmile Creek? Click here to view flood forecasting for other USGS stream stations in Montana .
Rain-driven flooding events can happen anytime during the spring so be prepared and watch the signs: streamflow, snowpack and weather. NOW is the time to start planning for spring flooding. Don't be caught off-guard.
Visit the Lewis & Clark County Flood Information Page for more information.
Additional Resources
Do you have any questions, or would you like to discuss a water resource issues with our staff? Please do not hesitate to contact us at:
City-County Building 316 N Park Avenue, Room 230 Helena, MT 59623
Jennifer McBroom, Supervisor (406) 457-8584 JMcBroom@LCCountyMT.gov
Kegan McClanahan, Hydrogeologist (406) 457-8927 KMcClanahan@LCCountyMT.gov
Natalie Emerson, Water Quality Specialist (406) 457 - 8585 NEmerson@LCCountyMT.gov
Here are some additional online resources:
Montana Bureau of Mines and Geology (MBMG)
United States Geological Survey (USGS)
