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Our Watershed


The mountainous upper portions of the Boulder St. Vrain watershed are primarily made up of granite and various metamorphic rocks over 1 billion years old. Meanwhile, the geology of the plains is primarily sedimentary rocks and deposits from river valleys that are much younger. Concentrations of various ions and metals reflect this transition. The hard rocks of the upper watershed do not weather as easily as the softer materials in the foothills and plains, and our streams naturally pick up some background concentrations of various metals as they move downstream. There are also deposits of various metals, such as gold, copper, and silver, in the upper watershed. Even if mining operations have ceased, old mines can leak heavy metals into our streams that are harmful to aquatic life.

Land Cover

The upper portions of the watershed are largely evergreen forest, with barren land and perennial ice near the Continental Divide and some areas of shrub interspersed throughout. In the lower foothills and plains this transitions to grassland, pasture, and cultivated crops and several large urbanized areas. There are areas of open water and various wetlands along Boulder Creek and St. Vrain Creek. Agricultural areas can contribute nutrients and bacteria to streams, and large urbanized areas contribute significant pollution in the form of stormwater runoff that contains various chemicals, nutrients, bacteria, and other pollutants. As land cover changes moving downstream we see concentrations of many pollutants increase from the background levels in their undeveloped headwaters.


Most of the water in our streams starts as snowmelt high in the mountains, which means there are some major seasonal trends in streamflow that affect water quality. Flows are generally highest in early summer when snow is melting and at their lowest during the fall before snow has accumulated. Higher flows can dilute certain pollutants, but they can also pick up additional pollutants through stormwater runoff, and it is important to consider stream flow and recent precipitation events when evaluating water quality. Our watershed also has many diversions that move water across watershed boundaries and distribute it for use. As a result, some streams may decrease in flow as you move downstream and the water at any given point may have taken some detours before arriving there.

Why Does Water Quality Matter?

Our watershed provides drinking water, recreation opportunities, habitat for aquatic life, and other important services. When water quality is degraded by various pollutants it can increase risk of illness when recreating, harm aquatic communities, and have other negative effects. Protecting water quality keeps our watershed healthy and thriving.

Water Quality in the Boulder St. Vrain Watershed

The Boulder St. Vrain watershed generally has good water quality but like all watersheds impacted by human activities it is not without its issues. Like streams in urbanized areas all across the country, several streams in our watershed elevated levels of Escherichia coli (E. coli) /(Spanish) intermittently throughout the year. Our watershed also has a long history of mining. Although most mines are no longer active and many are managed to reduce their impact, the excavated areas and their tailings can still leak metals into nearby waterbodies. There are several areas in the upper watershed with elevated metal concentrations that can harm aquatic life. Nutrients like nitrogen and phosphorus are present all around us, but human activities can lead to excess concentrations in our waterways. Nutrients from pet waste, lawn fertilizers, and detergents often end up in nearby streams and can be significant sources of pollution, producing algal growth that is harmful to aquatic life. Wastewater treatment plants (WWTPs) process our used indoor water, but cannot remove all of the nutrients that we flush down our sinks and toilets.

Join Your Neighbors in Taking Action

The things you do and choices you make every day have an impact on water quality. Many residents of our watershed are already taking steps and making small changes to their routine to protect water quality for those who live downstream from them. Here are some ways you can be a part of protecting water quality:

  • Pick up after your dog—on-leash, off-leash and in your yard. Learn more about the impacts of pet waste on our waterways.
  • Properly maintain your septic system. See Septic Smart for more information.
  • Secure garbage cans and pet food to reduce food sources for urban wildlife such as raccoons.
  • Tend to landscaping with organic practices and alternative pest management techniques to keep them healthy – healthy plants and lawns require less fertilizers and pesticides. Learn more about the impacts of yard waste and pesticides and fertilizers on our waterways.