Chapter 1: Introduction

1.2 Watersheds 101

A watershed is that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community.

—John Wesley Powell

A watershed is an area of land where all the water that flows across it drains to a common end point. Watershed boundaries are determined by topography and influenced by gravity. In order to begin to re-design our urban communities to be in balance with the watershed ecosystem, we first need to understand how a watershed works. When rain falls to the land, some water either: soaks into the soil and percolates back into the groundwater; gets absorbed by vegetation; evaporates into the atmosphere, or collects and runs off towards the nearest body of water. The physical force of the water flowing down hill picks up and carries with it particles of soil, leaves, and other debris. The faster this water moves, the more sediment and debris it carries away downstream. Sediment and other materials will end up getting deposited wherever the water slows down enough to give gravity a chance to pull these particles back down to the ground. The erosion of sediment from one place and deposition in another place shapes our land and water systems over time. Land use—particularly the percentage of impervious land cover—is one of the biggest factors influencing watershed health.

Figure 1-1 Watershed Procesess
Source: Everest International Consultants 2006

As water moves along its path, it interacts with everything it touches. It may exchange molecules with soil, vegetation, paved surfaces or trash affecting the suitability of the aquatic environment for plants, fish, birds, animals and other organisms. Because the native species within this system are specifically adapted to their natural environment and to each other, too much of an introduced substance or species can throw the whole system off track and may determine whether native species can thrive or even survive in their habitat. Aquatic life is an important component of the world’s web of genetic diversity. Each species exists because they serve some necessary function in the larger inter-related system we are all a part of. Their health is an indicator of the availability and cleanliness of the water supplies we all need for our own survival.