Big Thomson Watershed, CO

Big Thompson Watershed Forum selected Hydros to develop State of the Watershed reports in 2010, 2015, and 2021. The analysis included database QAQC, developing a conceptual understanding of the drivers of water-quality across the watershed, summarizing current water-quality patterns, evaluating changes in water quality over time through statistical trend testing, and comparing data to applicable Colorado water-quality standards. To support development of a detailed conceptual understanding, information on underlying geology, land use, population density, wastewater effluent, and water management (diversions and managed inflows) were compiled in addition to the observed water-quality data.

Parameters in the analysis included flow rates, metals (arsenic, copper, cadmium, lead, selenium, and zinc), nutrients (ammonia, nitrate+nitrite, total Kjeldahl nitrogen, total nitrogen, total phosphorus, and orthophosphate), microbiological parameters (total coliforms and E. coliforms), and a group of general parameters (alkalinity, dissolved oxygen, hardness, pH, specific conductivity, sulfate, temperature, total organic carbon, and total suspended solids).

Reports detailing the findings of the analyses for each five-year update were created in 2010, 2015, and 2021. Each report included an executive summary written to support communication of results to all stakeholders.  Key findings included:

• Identification of increasing trends in nutrient and total organic carbon concentrations in the Little Thompson River that appear to coincide with the recent population growth;

• Clear characterization of the water quality effects of trans-basin inflows from the Colorado-Big Thompson project;

• Evaluation of the observed effects of 2013 flooding, road reconstruction activities, and major wildfires on water quality in the watershed;

• Distinction between the upper and lower watersheds in terms of issues relative to water-quality standards, including the roles of geology and land use; and

• Recommendations related to data gaps in the existing sampling program to better support future data assessments.