What is a watershed (and what makes a healthy one)?

Written by Kelly Loria
Cover image by Michael Browning

Take a step outside and look around–you are currently in a watershed. Since water follows the path of least resistance as it ebbs downward from high to low areas, a watershed is any land that drains water or snow into a body of water like a stream, river, wetland, or lake.

Watersheds are defined by topographic features like mountains, and can refer to very large (like really, really big) stretches of land. For example, the Mississippi River is one of the largest watersheds within the Continental U.S. and receives water from 33 states.

Because watersheds can span across vast landscapes, water is often dirtied by feedlots, enriched with fertilizers, and contaminated by oils, trash, and anything else it can carry–which is a problem because this water becomes our drinking water, shower water, and toilet water, all of which have the potential to affect our health.

Therefore, it’s super important to keep watersheds clean–but how do we know if a watershed is healthy enough to provide clean water? The answer: a list of healthy watershed characteristics created by the Environmental Protection Agency:

  1. “Dynamic hydrologic and geomorphologic processes within their natural range of variation.” 

    At first this sounds like gibberish, but broken down, it means a little something like this: Aquatic environments evolved to handle seasonal patterns of precipitation. This means that aquatic ecosystems need small, regular floods that submerge riverside floodplains to create sandbars, channels, and diverse aquatic habitats, which provide valuable nutrients for organisms and vegetation. For a watershed to remain ecologically intact so that it can provide economically valuable commodities and services to us hooomans, it needs to receive enough water and at the right times. Once this happens, we can extract water for our own needs without causing too much harm to the watershed. 

  1. “Habitat of sufficient size and connectivity to support native aquatic and riparian species.”

    When we build dams or water diversions, we block aquatic species from traveling around as much as they once did. This is called habitat fragmentation, which makes it challenging for certain species to find food and mates, or for migratory species to find places to rest and feed along their routes. Modern updates to aquatic infrastructure, such as fish ladders that connect rivers to reservoirs with water steps, can help mediate some of this fragmentation and facilitate aquatic species travel.

  1. “Physical and chemical water quality conditions able to support healthy biological communities.”

    Water quality is assessed based on both its chemistry (i.e. how much nitrogen is in the water) and physical features (i.e. temperature). The level to which water quality is monitored and enforced varies state by state, which means that states can have a regulatory standard that is lower than what is best for maintaining healthy aquatic life. This is important because whatever happens upstream–like fertilizer use–can have really harmful impacts on aquatic ecosystems in downstream. 

What happens upstream matters. When aquatic systems receive high concentrations of nutrients like nitrogen and phosphorus, it can cause rapid growth of algae (HABs aka Harmful Algal Blooms). Algae exists in all aquatic environments, but when they come in contact with nitrogen and phosphorus from outside of their natural habitats (like from fertilizers) they grow and grow and grow some more. When the algae eventually dies, the process of breaking down the huge amount of dead algae lowers the amount of oxygen available for other aquatic life, resulting in poor water quality. This is a huge problem for the Mississippi because it has 33 states worth of agricultural nitrogen and phosphorus to pick up before it pours into the Gulf of Mexico, which causes the infamous dead zone.  

Watersheds are very cool and are the whole reason we have water–but, not every watershed meets these EPA standards, resulting in contaminated water. That’s where you, me, and all of our friends come in. It’s our job to advocate for clean water. So, like always, contact them reps, baby!

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