Mason Creek, Oconomowoc River Watershed (UR09)
Mason Creek, Oconomowoc River Watershed (UR09)
Mason Creek (851100)
2.03 Miles
4.11 - 6.14
Natural Community
Natural communities (stream and lake natural communities) represent model results that use predicted flow and temperature based on landscape features and related assumptions. Ranges of flow and temperature associated with specific aquatic life communities (fish, macroinvertebrates) help biologists identify appropriate resource management goals. Wisconsin Natural Communities.
Coldwater, Cool-Warm Headwater
Year Last Monitored
This is the most recent date of monitoring data stored in SWIMS. Additional surveys for fish and habitat may be available subsequent to this date.
This river is impaired
Low DO, Elevated Water Temperature
Total Phosphorus, Sediment/Total Suspended Solids
Trout Water 
Trout Waters are represented by Class I, Class II or Class III waters. These classes have specific ecological characteristics and management actions associated with them. For more information regarding Trout Classifications, see the Fisheries Trout Class Webpages.
Outstanding or Exceptional 
Wisconsin has designated many of the state's highest quality waters as Outstanding Resource Waters (ORWs) or Exceptional Resource Waters (ERWs). Waters designated as ORW or ERW are surface waters which provide outstanding recreational opportunities, support valuable fisheries and wildlife habitat, have good water quality, and are not significantly impacted by human activities. ORW and ERW status identifies waters that the State of Wisconsin has determined warrant additional protection from the effects of pollution. These designations are intended to meet federal Clean Water Act obligations requiring Wisconsin to adopt an 'antidegradation' policy that is designed to prevent any lowering of water quality - especially in those waters having significant ecological or cultural value.
Impaired Water 
A water is polluted or 'impaired' if it does not support full use by humans, wildlife, fish and other aquatic life and it is shown that one or more of the pollutant criteria are not met.

Fish and Aquatic Life

Current Use
The use the water currently supports. This is not a designation or classification; it is based on the current condition of the water. Information in this column is not designed for, and should not be used for, regulatory purposes.
Streams capable of supporting a warm waterdependent forage fishery. Representative aquatic life communities associated with these waters generally require cool or warm temperatures and concentrations of dissolved oxygen that do not drop below 5 mg/L.
Attainable Use
The use that the investigator believes the water could achieve through managing "controllable" sources. Beaver dams, hydroelectric dams, low gradient streams, and naturally occurring low flows are generally not considered controllable. The attainable use may be the same as the current use or it may be higher.
Cold (Class II Trout)
Streams supporting a cold water sport fishery, or serving as a spawning area for salmonids and other cold water fish species. Representative aquatic life communities, associated with these waters, generally require cold temperatures and concentrations of dissolved oxygen that remain above 6 mg/L through natural reproduction and selective propagation. Since these waters are capable of supporting natural reproduction, a minimum dissolved oxygen concentration of 7 mg/L is required during times of active spawning and support of early life stages of newly-hatched fish.
Designated Use
This is the water classification legally recognized by NR102 and NR104, Wis. Adm. Code. The classification determines water quality criteria and effluent limits. Waters obtain designated uses through classification procedures.
Default FAL
Fish and Aquatic Life - Default Waters do not have a specific use designation subcategory but are considered fishable, swimmable waters.


All of the stream in Waukesha County.

Date  2002

Author   Aquatic Biologist


Mason creek originates in section 6 of Merton Township at the confluence of its east and west branches. These branches begin in Erin Township (Washington County) and are extensively ditched for agricultural drainage. The east branch is about one mile long, the west branch about two miles. The mainstem is 5.2 miles, has a relatively shallow gradient (6-7 feet per mile) and empties directly into North Lake. Historically, Mason Creek supported a brook trout fishery. Brook trout are found in Mason Creek today but their distribution fluctuates.

The west branch of Mason Creek experiences low dissolved oxygen concentrations (below 6 ppm; saturation 54-67 %), turbid water, siltation of substrate, and temperatures up to 75 degrees Fahrenheit. In the east branch, water temperature and dissolved oxygen concentrations are considered excellent for trout. A gravel substrate is present and could be suitable for trout reproduction.

The mainstem near the confluence of the east and west branches has good water quality for brook trout. Fish collections have included over 50 brook trout; forage fish on the other hand are dominated by pollution-tolerant species. In 2000, a four-mile stretch of the east branch and main stem of Mason Creek was classified as a Class I trout stream.

Upstream of County Trunk Highway CW, dissolved oxygen concentration, flows and cover are excellent for trout, but high temperatures (74 F) and silted substrate may be limiting trout populations. A recent fish collection showed only 12 brook trout, a forage fish population dominated by types of pollution tolerant fish, and one intolerant species (pearl dace).

Between 1973 and 1983, WDNR water resources managers extensively monitored the lower mainstem of Mason Creek at Shore Drive (just below County Trunk Hwy. CW); and at Krebster Road and County Trunk EC (above North Lake). Though dissolved oxygen concentrations ranged from 7.0 to 10 ppm, water temperatures were considered marginal for supporting trout. Phosphorus concentration ranged from 0.02 to 0.99 ppm. Gravel substrate was found at the three stations, but significant siltation was also noted.

Fish collections were conducted in the lower mainstem of Mason Creek in 1975, 1981 and 1983. In 1975, fish were collected at County Trunk Hwy. EC representing 14 species. Pollution-intolerant forage species dominated the fish community. The slender madtom, an state endangered catfish, was also found at this site. During 1981, the entire lower section of the mainstem was surveyed. No brook trout were found, but juvenile smallmouth and largemouth bass, hornyhead chub, rainbow darters and central stonerollers existed. In 1983, collections at County Trunk Highway EC found 13 forage species, with nine pollution-tolerant species.

Bacteriological sampling conducted at the County EC bridge showed counts for a type of bacteria (MFFC) ranging from <10 per 100 ml to 11,000 per 100 ml. About 40 percent of the samples exceeded 400 MFFC per 100 ml, the state water quality standard.

Date  2002

Author   Aquatic Biologist

Mason Creek, Oconomowoc River Watershed (UR09) Fish and Aquatic LifeMason Creek, Oconomowoc River Watershed (UR09) RecreationMason Creek, Oconomowoc River Watershed (UR09) Fish Consumption

Impaired Waters

The 2018 assessments of Mason Creek (miles 0-4.11) showed continued impairment by temperature; new temperature sample data exceeded the 2018 WisCALM listing criteria for the Fish and Aquatic Life use. Based on the most updated information, no change in the existing impaired waters listing was needed.

Date  2017

Author  Ashley Beranek


Wisconsin has over 84,000 miles of streams, 15,000 lakes and milllions of acres of wetlands. Assessing the condition of this vast amount of water is challenging. The state's water monitoring program uses a media-based, cross-program approach to analyze water condition. An updated monitoring strategy (2015-2020) is now available. Compliance with Clean Water Act fishable, swimmable standards are located in the Executive Summary of Water Condition in 2018. See also the 'monitoring and projects' tab.



Citizen-Based Stream Monitoring
Collect chemical, physical, and/or biological water quality data to assess the current overall stream health. The data can inform management decisions and may be used to identify impaired waters for biennial lists.
Monitor Water Quality or Sediment
2013 TP: "May Meet". AU: 11498. Station 10038441. Miles 0 - 4.11. Potential TP delisting in 2020.
Monitor Water Quality or Sediment
Category 4A. Confirmed with new temperature data. 2018 TP Results: May Meet. Station: 10038441. AU: 11498.
Monitor Targeted Area
Mason Creek Planning Program The North Lake Management District will acquire data on Mason Creek channel width, depth, sinuosity and length, habitat descriptors, biological and chemical water quality and input into DNR\2019s stream and lake monitoring program. The project deliverables include the following: 1) A copy of the completed plan which will identify strategic land and water resource management actions, specific interventions, such as streambank stabilization, buffer implementation and wetland restoration. 2) Results of an informational hearing.
TMDL Monitoring
The purpose of the project is to monitor and evaluate portions of Mason Creek and its tributaries in Waukesha and Washington Counties. Mason Creek is currently 303d listed for pollutants including total phosphorus and sediment/total suspended solids, with impairments including low DO, degraded habitat and elevated Temperatures and yet it is still listed as a Class I Trout stream. It is also part of the Rock River TMDL area.

Management Goals

Wisconsin's Water Quality Standards provide qualitative and quantitative goals for waters that are protective of Fishable, Swimmable conditions [Learn more]. Waters that do not meet water quality standards are considered impaired and restoration actions are planned and carried out until the water is once again fishable and swimmable

Management goals can include creation or implementation of a Total Maximum Daily Load analysis, a Nine Key Element Plan, or other restoration work, education and outreach and more. If specific recommendations exist for this water, they will be displayed below online.


Monitoring the condition of a river, stream, or lake includes gathering physical, chemical, biological, and habitat data. Comprehensive studies often gather all these parameters in great detail, while lighter assessment events will involve sampling physical, chemical and biological data such as macroinvertebrates. Aquatic macroinvertebrates and fish communities integrate watershed or catchment condition, providing great insight into overall ecosystem health. Chemical and habitat parameters tell researchers more about human induced problems including contaminated runoff, point source dischargers, or habitat issues that foster or limit the potential of aquatic communities to thrive in a given area. Wisconsin's Water Monitoring Strategy was recenty updated.

Grants and Management Projects

Monitoring Projects

Watershed Characteristics

Mason Creek is located in the Oconomowoc River watershed which is 130.86 miĀ². Land use in the watershed is primarily agricultural (24.90%), forest (19.70%) and a mix of wetland (16.90%) and other uses (38.40%). This watershed has 136.99 stream miles, 2,858.66 lake acres and 11,105.19 wetland acres.

Nonpoint Source Characteristics

This watershed is ranked Not Available for runoff impacts on streams, Not Available for runoff impacts on lakes and High for runoff impacts on groundwater and therefore has an overall rank of High. This value can be used in ranking the watershed or individual waterbodies for grant funding under state and county programs.However, all waters are affected by diffuse pollutant sources regardless of initial water quality. Applications for specific runoff projects under state or county grant programs may be pursued. For more information, go to surface water program grants.

Natural Community

Mason Creek is considered a Coldwater, Cool-Warm Headwater under the state's Natural Community Determinations.

Natural communities (stream and lake natural communities) represent model resultsand DNR staff valiation processes that confirm or update predicted conditions based on flow and temperature modeling from historic and current landscape features and related variables. Predicated flow and temperatures for waters are associated predicated fish assemblages (communities). Biologists evaluate the model results against current survey data to determine if the modeled results are corect and whether biological indicators show water quaity degradation. This analysis is a core component of the state's resource management framework. Wisconsin's Riverine Natural Communities.

Cool (Warm-Transition) Headwaters are small, sometimes intermittent streams with cool to warm summer temperatures. Coldwater fishes are uncommon to absent, transitional fishes are abundant to common, and warm water fishes are common to uncommon. Headwater species are abundant to common, mainstem species are common to absent, and river species are absent.

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