What is the Integrated Report?

Every two years, Sections 303(d) and 305(b) of the federal Clean Water Act require Colorado to report on the quality of its waters to the U.S. Environmental Protection Agency. Colorado combines both requirements in one Integrated Report, which gives the attainment status of every surface water in the state.

This page applies the same reporting framework to the waters the NFRWQPA tracks for its 208 water quality management plan, limited to Larimer and Weld counties.

How it fits the Clean Water Act

Colorado sets water quality standards to protect each water's classified uses: aquatic life, recreation, water supply and agriculture. Section 303(d) requires the state to evaluate all available data and list the waters that do not meet those standards. Listed waters stay on the list, and keep being monitored, until they meet their standards.

The 303(d) List and the Monitoring and Evaluation List

The 303(d) List of Impaired Waters names waters that do not meet standards and need a total maximum daily load (TMDL) or other restoration plan. The Monitoring and Evaluation (M&E) List flags waters that may have a problem where the data are uncertain. The Water Quality Control Commission adopts both lists in Regulation 93.

How waters come off the 303(d) List

A water or pollutant is removed when new data show the standard is being met, when the EPA approves a TMDL or alternative plan, or when standards or assessment methods change. In 2026 the Division removed pollutant listings from waters in this study area:

From Appendix C (Delisting Table) of the Division's 2026 Integrated Report, filtered to study-area assessment units.

The Division places each assessment unit in one category. Categories are first applied to each pollutant and use; when a water has several, it takes the highest (least attaining) category.

Categories run from Category 1, where a water meets standards for all its classified uses, to Category 5, where at least one use is not met and a TMDL is needed.

The study area

The NFRWQPA 208 area drains the Front Range from Rocky Mountain National Park east across the plains of Larimer and Weld counties. Its waters belong to five subbasins of the South Platte regulatory basin: the Big Thompson, Cache la Poudre, St. Vrain, Big Dry Creek and the South Platte mainstem.

This page counts the assessment units from the workbook that have at least 5% of their length or area inside Larimer or Weld County. Sizes are the in-county portion of each unit.

Waterbody IDs and assessment unit IDs

Colorado identifies segments with waterbody IDs such as COSPBT02. CO is the state, SP the South Platte basin, BT the Big Thompson subbasin, and 02 the segment. Segments are split into assessment units for reporting, shown by a suffix such as _A: COSPBT02_A is one portion of segment 2.

Black outlines are Larimer and Weld counties. Click a stream or lake for its details.

Each assessment unit is colored by its 2026 reporting category. The charts follow the Division's reporting categories dashboard: stream miles and lake acres in each category.

When a standard is exceeded, the use it protects is in non-attainment, and the pollutant is the cause. In the study area the most common causes for rivers and streams in 2026 are , measured by miles on the 303(d) List.

Arsenic dominates statewide as well. Colorado's arsenic water supply standard is set at a very low, human-health-based level, and many listings reflect naturally occurring arsenic.

Bars show the in-county miles of assessment units listed on the 303(d) List for each pollutant; one unit can be listed for several pollutants. The count is the number of units.

For lakes in the study area, the most common causes in 2026 are , measured by acres on the 303(d) List.

Two causes are new in 2026: chlorophyll a at Milton Reservoir (COSPMS04_B) and Prospect Lake (COSPMS07_B), and dissolved manganese at Horsetooth Reservoir (COSPCP14_A).

Fully supporting waters, 2018–2026

Bars split fully supporting waters into units already assessed in 2018 and units added or resegmented later. The line shows the share of all assessed waters that were fully supporting.

Fully supporting by classified use

Where the change came from

Cycle-to-cycle totals mix three things: waters restored from impaired, waters confirmed as supporting the first time they were assessed, and waters added to the network. This breakdown separates them for each use.

Status of all reported waters

The same streams since 2002

When each pollutant first appeared

The first cycle each pollutant was listed on a panel segment. Arsenic and temperature, the two causes behind most of the change after 2008, arrived with new standards and assessment methods rather than with a known change in land use or discharges.

The Division notes that water quality status is hard to track from cycle to cycle because listing methods, table value standards and the amount of monitoring all change. The NFRWQPA findings below handle that by following the same assessment units through every cycle from 2018 to 2026.

1. The assessed network grew

In-county assessed stream miles rose from in 2018 to in 2026 as segments were split and new assessment units were added. Region-wide totals from different cycles describe different stream networks, so they can't be compared directly.

2. On the same waters, conditions held steady

The 2018→2020 shift mostly reflects Category 2 waters (some uses not yet assessed) getting full assessments, not a change in the water.

3. Listing changes, cycle by cycle

Pollutant listings added and removed on assessment units present in both cycles (303(d) and TMDL listings; M&E excluded).

4. What changed in 2026

Improvements. Total mercury came off the 303(d) List on nine Big Thompson assessment units, and copper, zinc, iron, nitrate, selenium, pH and macroinvertebrates came off others. The Division gives "data attaining" as the reason for of the study-area delistings. COSPBT04_A (Big Thompson, County Road 11H to I-25) moved from Category 5 to 1a, and Horse Creek Reservoir (COSPMS07_C) left the 303(d) List when pH was removed, moving to Category 3b.

New listings. Temperature on six stream units, arsenic on five, chlorophyll a at Milton Reservoir and Prospect Lake, manganese at Horsetooth Reservoir and on Buckhorn Creek (COSPBT07_A), and dissolved iron on Buckhorn Creek. Temperature and arsenic listings often follow new data or standards. Chlorophyll a and manganese in reservoirs deserve a closer look.

5. Change since 2018, unit by unit

Each unit is colored by how its pollutant listings changed between its first assessment and 2026. Maintained means the same listings. Units created after 2018 are shown in gold.

Largest declines and their likely causes

Causes are first-pass groupings by pollutant and need case-by-case review.