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Learn the Basics About
Watersheds & Stormwater

Map of the Boulder St. Vrain watershed

What is a Watershed?

A watershed is an area of land where all precipitation and streams drain to a single point. Watersheds can be extremely large, draining water from numerous states, or can be divided into smaller subwatersheds that cover only a few square miles. In the Front Range of Colorado most of our water originates as snow high up in the Rocky Mountains. Some of this water will be diverted and stored for drinking water, agriculture and other uses rather than traveling downstream, but our streams and return flows all come together to a single point where our watershed ends and becomes part of a bigger watershed.

The Boulder St. Vrain watershed covers most of Boulder County, several towns in Weld County, and small portions of a few other counties. Since water doesn’t flow according to political boundaries, it’s important for all of us to work together to protect our watershed.

Our Watershed Connects Our Community

Everyone lives in a watershed. It may be hard to see if you do not live near a waterway, but water is moving through streams, ditches, and underground pipes through our communities. As water travels downstream it is impacted by human activities as it passes through cities, towns, and farms.

How the communities upstream from you treat the water will impact your water use, and how you treat your water will impact the communities downstream from you. Trash and polluted stormwater runoff from your neighborhood will end up in the stream people downstream from you use to fish and swim.

A healthy community starts with a healthy watershed. We all depend on our watersheds for clean drinking water, productive farms, thriving aquatic life, and safe places to swim, fish, and paddle. We need to keep our waterways clean for our community – and the communities who live downstream from us.

Stormwater: A Major Source of Water Pollution

Stormwater pollution occurs when rain or snow melt flows over streets and picks up trash, oil, dirt, and other pollutants as it travels. These pollutants are then carried through the storm drainage system, which drains directly into our local creeks and streams untreated. The stormwater system differs from the wastewater system, which carries water from indoor drains in our homes, offices, and buildings, to wastewater treatment plants.

Infografía en español.

Infographic entitled "Where Does My Water Go?" describes the stormwater and wastewater systems. Stormwater enters storm drains along roadways and travels through underground pipes, draining directly into local waterways. Wastewater used in our homes goes through separate pipes to the wastewater treatment plant, where it is cleaned before entering back into local waterways.

Common Stormwater Pollutants

Yard Waste

Grass clippings, fallen leaves, and other yard waste contain nutrients that are great for composting but can cause harmful algal blooms in waterways.

Pet Waste

Pet waste contains harmful bacteria, parasites, and nutrients that do not belong in our waterways. 

Fertilizers and Pesticides

Pesticides and fertilizers applied to lawns, parks, and other areas are toxic to aquatic life and can cause harmful algal blooms.

Household Waste

Household hazardous waste includes products that contain toxic chemicals that are dangerous to people and harmful to water quality and aquatic life.

Car Maintenance

Cars can leak motor oil, antifreeze, and other chemicals that are toxic to aquatic life and the wildlife that drink from our streams.


Litter pollutes our waterways, harms wildlife, and clogs stormwater infrastructure.


For Residents

Residents of any watershed can act daily to protect waterways and reduce their impact on the stormwater system.

For Businesses

All businesses can take action to protect waterways and specific businesses can directly impact our creeks and streams.

For Teachers

Find educational resources for inside and outside the classroom and opportunities for free programming in our watershed.

For Stormwater Professionals

Training and other materials support stormwater, construction, and maintenance professionals in protecting waterways.