Watershed Management

What is a watershed?

A watershed is an area of land that drains rainwater or snow into one location, like a river, lake, or ocean. It is the region where all the water that falls within it flows to a common outlet. Watersheds vary in size from small areas that drain into a stream to large regions that encompass multiple rivers and lakes (see right).

Watersheds are critical for:

Ecosystem Health: Watersheds support diverse ecosystems and habitats for various plant and animal species.
Flood Control: They play a role in managing stormwater and reducing flooding by absorbing and slowing runoff.
Water Quality: Healthy watersheds help filter pollutants and sediments, improving the quality of water that flows into waterbodies.
Water Supply: They provide fresh water for drinking, irrigation, and industrial use.

Graphic depicting a watershed (Source: NOAA)

What is a total maximum daily load (TMDL)?

A TMDL is a guide used to restore water quality in specific rivers, lakes, or bays that do not meet water quality standards. Acting as a “pollution diet”, TMDLs establish the calculated maximum amount of a pollutant that a waterbody can receive without exceeding the water quality standard for that pollutant. Regulatory agencies develop TMDLs for individual waterbodies or watersheds. They take this approach because pollutants can originate from various sources within the watershed and travel to the waterbody. Therefore, developing a TMDL requires understanding the sources of pollution within the watershed and how they impact the waterbody.

A TMDL addresses a single pollutant or stressor for each waterbody. It is represented as a mass per unit of time (e.g., pounds per day), which is known as the load. A TMDL might indicate, for example, that a maximum load of 1,000 pounds of sediment per day could enter a stream before the stream experiences negative effects. The TMDL for a given pollutant and waterbody includes the sum of that pollutant from individual point sources (i.e., wasteload allocations [WLAs]) and from nonpoint sources (i.e., load allocations [LAs]). The WLA is the portion of the TMDL assigned to permitted dischargers, such as the County’s Municipal Separate Storm Sewer System (MS4).

The County’s National Pollutant Discharge Elimination System (NPDES) MS4 permit requires that the County develop local watershed restoration plans to address each U.S. Environmental Protection Agency (EPA)-approved TMDL with stormwater WLAs.