Big Dry Creek Watershed Highlights

The Big Dry Creek basin originates in unincorporated Jefferson County at the mouth of Coal Creek Canyon at an elevation of approximately 8,000 feet above sea level. The total drainage area is approximately 110 square miles.  The headwaters area is located approximately 5 miles west of Rocky Flats and drains to the east across Rocky Flats, where several tributaries including Walnut Creek, Woman Creek, and Upper Big Dry Creek form above Standley Lake.  Much of the drainage from this area is routed around Standley Lake to Walnut Creek.  Standley Lake receives water conveyed in ditches from the Clear Creek basin. 

The main stem of Big Dry Creek begins at the Standley Lake outfall and flows in a northeasterly direction approximately 33 miles to its confluence with the South Platte River near Fort Lupton in Weld County.  Historically, Interstate 25 roughly divided the watershed between urban and agricultural land uses with agricultural uses dominating east of I-25; however, development has spread east of I-25 over the past 20 years, particularly as transportation infrastructure such as E-470/Northwest Parkway has been implemented and future plans for light rail extensions move forward.  The confluence of Big Dry Creek and the South Platte River marks the downstream end of Segment 15 of the South Platte River.

The Big Dry Creek basin is outlined in the map to the right, along with the municipalities and counties comprising the watershed.  Significant portions of the watershed are currently undergoing or have recently urbanized, transitioning from predominantly agricultural uses to include a mixture of residential, commercial and industrial uses.  The basin is significantly influenced by Standley Lake reservoir located in the upper basin, as well as by discharges from the Broomfield and Westminster municipal wastewater treatment plants (WWTPs) in the central portion of the basin.  Stormwater runoff in the urbanized portion of the basin, ditch diversions and agricultural return flows in the lower watershed also significantly influence stream characteristics.  Northglenn’s WWTP discharges to Big Dry Creek in Weld County.  The majority of its discharges are to Bull Canal, although direct discharges to Big Dry Creek have increased significantly over the last several years and will continue to increase.

The Water Quality Control Commission establishes stream classifications and standards for waterbodies throughout Colorado. Currently applicable standards in place to protect aquatic life, recreation, agriculture and water supply can be found in  Regulation 38  for Big Dry Creek Segment 1, which extends from the Standley Lake Dam to the South Platte River near Fort Lupton. Numeric standards include general water chemistry parameters, E. coli, nutrients and metals.

The main stem of Big Dry Creek meets most of the stream standards assigned by the Water Quality Control Commission, but several pollutants exceed water quality standards. Some pollutant concentrations are expected to be elevated due to natural sources, whereas urban and agricultural activities influence other pollutants. 

In 2024, BDCWA completed an evaluation of the past five years of water quality data collected for the watershed from 2019 to 2023. The full report can be accessed at  https://www.bigdrycreek.org/reports . Highlights for several key water quality constituents that BDCWA monitors are provided below. Big Dry Creek currently meets all of the water quality standards assigned for the protection of aquatic life and agricultural use. Big Dry Creek does not currently meet recreational water quality criteria, future anticipated nutrient criteria for phosphorus and nitrogen, and several secondary drinking water standards that are not human health-related.

E.coli

E.coli is a fecal indicator bacteria that is used in EPA's Recreational Water Quality Criteria and adopted by CDPHE as a stream standard of 126 colony-forming units per 100 milliliters (CFU/100mL). E. coli can originate from both human and non-human sources, with non-human sources such as birds expected to pose a lower human health risk than sources such as humans and cattle.

CDPHE established a Total Maximum Daily Load (TMDL) for Big Dry Creek in 2016 to target reductions in E.coli needed to meet the stream standard. BDCWA has 24 years of E.coli data collected monthly at eight instream locations.

E. coli concentrations are elevated at multiple instream locations. E. coli concentrations in the WWTP discharges are very low and do not exceed stream standards. Efforts are underway to identify sources of E. coli upstream of I-25. The current monitoring program is useful for identifying elevated stream reaches and trends over time but is not sufficient for identifying and mitigating sources of E. coli. For this reason, BDCWA and partnering cities have conducted a microbial source identification study on the creek and dry weather outfall sampling. Locations with the most elevated E. coli occur at I-25 (bdc3.0) and in an agricultural area in Weld County (bdc6.0).

Total Phosphorus

Phosphorus is a nutrient present in the environment in various forms.  High levels of phosphorus in waterbodies can lead to excessive algae growth, which can deplete oxygen levels, harm aquatic life, and potentially produce toxins. In 2012, the Colorado Water Quality Control Commission adopted interim nutrient criteria for total phosphorus (and total nitrogen), which are expected to become effective for the main stem of Big Dry Creek in 2030. Additionally, Big Dry Creek has been assigned a phosphorus Load Allocation in a downstream Total Maximum Daily Load (TMDL) for Barr Lake and Milton Reservoir and has been assigned a phosphorus load reduction.

Phosphorus concentrations and loads to Big Dry Creek have decreased significantly over time as a result of WWTP upgrades at the Broomfield and Westminster WWTPs, along with reuse programs that continue to be implemented at these WWTPs. Average phosphorus concentrations at bd6.0 in the lower watershed have decreased by 77 percent since 2003. Despite these substantial improvements, the stream does not yet meet the interim total phosphorus criteria (annual median of 0.17 mg/L) from below the Westminster WWTP to the confluence with the South Platte River.

Total Nitrogen

Nitrogen is present in the environment in various forms.  Excessive amounts in waterbodies can lead to harmful algal blooms, deplete dissolved oxygen levels, and negatively impact aquatic life. Total nitrogen is calculated based on total Kjeldahl nitrogen (TKN) plus nitrate/nitrite or through direct analysis of total nitrogen. TKN includes organic nitrogen and ammonia. CDPHE’s interim total nitrogen value is 2.01 mg/L, assessed as an annual median. Big Dry Creek does not attain the warm water instream nitrogen “interim value” below municipal WWTP discharges (from the Broomfield WWTP to the South Platte River). Although the nutrient interim values are not expected to be adopted as stream standards on the main stem of Big Dry Creek before 2030, addressing nutrient sources to Big Dry Creek continues to be an area of focus for BDCWA and for WWTPs in the watershed. Municipal WWTPs discharging to Big Dry Creek have compliance plans to achieve total inorganic nitrogen limits.

Total Recoverable Iron

Big Dry Creek is listed on CDPHE’s 303(d) List of impaired waters for total recoverable iron.  For the 2019-2023 timeframe, BDCWA’s five-year data set meets the iron standard of 1 mg/L for the stream as a whole and for each monitoring location. Additionally, bdc6.0 at Weld County Road 8 shows attainment of the iron standard with a median of 0.6 mg/L for 2019-2023. Metro Water Recovery’s independent monitoring of the lower watershed also demonstrated attainment of the standard for this timeframe. The expected source of elevated iron is streambank and soil erosion in the watershed, often in response to storm events. Previous analyses by BDCWA have shown that total iron and total suspended solids (TSS) are highly correlated, with both concentrations tending to be elevated during storm events. Attainment of the total recoverable iron standard in the lower watershed is expected to vary year-to-year, depending on the timing of sampling events relative to storm events.

Secondary Drinking Water Parameters

Sulfate, chloride, manganese and dissolved iron are secondary drinking water parameters that EPA established as guidelines to assist public water systems in managing their drinking water for aesthetic considerations, such as taste, color, and odor. These contaminants are not considered to present a risk to human health. Although there are no public water supply intakes on Big Dry Creek, there are alluvial wells in the agricultural portion of the watershed that are used for drinking water. CDPHE added a Water Supply classification to Big Dry Creek in 2020 because of these wells.

Sources of sulfate, chloride and dissolved manganese in the watershed include groundwater inflows in the upper watershed, as evidenced by seasonal patterns in the data set. Other sources of sulfate may also be present in the lower watershed, but at levels below the stream standard. All of these constituents are expected to have exceedances of the stream standards potentially resulting in impairment listing on future CDPHE 303(d) lists. For chloride, the most recent 5-year period does not meet the stream standard; additionally, a trend of increasing chloride concentrations over time is suggested by the data, which may include multiple sources in addition to groundwater. BDCWA has not fully evaluated sources of chloride in the watershed but is following the work of the South Platte Salinity Work Group on this issue.

In the upper portion of Big Dry Creek, BDCWA provides financial support to maintain the USGS streamflow gauge behind Front Range Community College. A USGS gauge is also located downstream near Fort Lupton. Click below to access the latest flow data:

A Colorado Division of Water Resources gauge is also located in the lower watershed above the Lupton Bottom diversion. Get the latest flow data:

The Westminster WWTP is called the “Big Dry Creek Wastewater Treatment Facility.”  The facility is an activated biosolids system consisting of aeration basins, secondary clarifiers, and ultraviolet disinfection.  The permitted capacity of the facility is 11.9 MGD, with 10 MGD of the treated effluent eligible for reclaimed water irrigation usage. The facility completed an expansion project in 2005 to improve treatment and expand capacity. The design capacity is 11.9 MGD, with 21,500 lbs of BOD for organic loading (CWQCD 2013).  The facility primarily discharges to Big Dry Creek at discharge point 004A, has the capability of also discharging from 003A, along with the Farmer’s Highline Canal at discharge point 001A, and to a variety of reclaimed water irrigation sites from point 005A.  Westminster has an approved industrial pretreatment program.

The City of Westminster owns and operates a Reclaimed Water Treatment Facility (RWTF), which began operation in 2000, with growth of the program continuing over the past 20 years. The RWTF is located just south of the BDCWWTF and receives final effluent for further treatment prior to distribution throughout the City’s reclaimed water system. The RWTF has a maximum capacity of 10 MGD. The RWTF typically uses all BDCWWTF effluent during the irrigation season except for approximately 50 gpm which is discharged to Big Dry Creek in order to avoid a zero-discharge situation. The facility was upgraded in 2009 to include a new 2.2 MG influent storage reservoir (HDR 2018). From the reservoir, flow is conveyed to the RWTF. The reclaimed water is used to irrigate large public turf areas such as golf courses, parks and greenbelts.