Introduction statement:

" Through the water we are all connected – a phrase Polk Soil & Water Conservation District references daily. The reason we say this isn’t just because we all (people, animals, plants) need water to survive, but also because each and every person is part of system called a watershed. No matter where you are, if you are standing on the ground on planet Earth you are in a watershed.

What is a watershed?

Watershed: An area of land in which all streams and rainfall drains to a common outlet like the outflow of a reservoir, the mouth of a river, or any point along a stream channel. The terms drainage basin, river basin, and catchment are also used to reference watersheds.

A watershed boundary is defined by the ridges of the area that topographically appears to drain water to a common point. A watershed can be as small as the area of two adjoining hillsides or it can be a larger area in which all the water drains to a single point like a lake. In Polk County, Iowa our local watersheds are part of larger watersheds that together form the Mississippi watershed that drains into the Gulf of Mexico.

Understanding watersheds is critical to understanding stream flow, water quality, and how to effectively manage water resources in sustainable ways. Anything happening in the land-area of a watershed affects the quantity and quality of water in that watershed.

Here is a look at the Rocky Mountains in Colorado along Ten Mile Creek.
Now look at this photo with some of the watersheds defined. As you can see, each watershed enters Tenmile Creek at a point. Following the ridges of the mountains from that point defines individual watersheds. Each watershed can be divided into smaller watersheds (see far right watershed)

Components of a watershed system

A watershed consists of surface water including streams, lakes, wetlands, reservoirs and groundwater. However the quality and quantity of stream and groundwater flow in a watershed is dependent on geology, soils, topography, land use, and climate.


How water flows throughout a watershed depends on the land – how it’s shaped, what it’s made of, and how the land is used. What direction is the water flowing? Is the water flowing slowly through thick grasslands and temporarily stored in a wetland? Is rainwater infiltrating through the soil recharging groundwater aquifers? How is this water running across an agricultural field? Is water rapidly flowing off buildings and parking lots into storm drains? Read more below to learn how different components of the land affect water flow.

  1. Soil – In Iowa, healthy soils function like a sponge helping to naturally infiltrate and percolate water into the soil profile. This process helps store water, recharges aquifers, and naturally filters the water. When damaged through compaction or over use, soil loses its structure and ability to store water leading to increased surface water runoff.

Across Iowa there over 450 soil subtypes each of which has different characteristics affecting how water flows. Certain soil types such as clay are very compact and have poor hydraulic conductivity (the rate at which soil is able to transport water), while other soils are easily eroded by runoff. Soil types help inform us of how water will flow and help us to strategically place development, and manage resources.


Resources: Iowa State Extension soil and land use web page:

Iowa Soil Regions Map: (pdf)

  1. Vegetation – Historically Iowa was covered by tallgrass prairie. The prairie, made up of dense grass and forbes with deep fibrous roots helped to slow down and store water in the soil naturally filtering water. As vegetation changes or is removed, there is an impact to how water will flow and the quality of that water, overall altering the watershed.

Think about a grassland or prairie – is there any exposed soil? Now think about a construction site or agricultural field in winter or early spring, how much soil is exposed? The amount of exposed soil has a direct impact on the amount of erosion that will occur on that landscape. Unless treated, exposed soil is eroded and carried away by water impacting water quality and land within the watershed.


Urban areas tend to have higher bacteria levels than agricultural areas