“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?
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.
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).
Iowa is split up into many different watersheds. These watersheds are a part of the larger Mississippi watershed, which eventually drains into the Gulf of Mexico. This means, that whatever we put into our water systems in Polk County ends up in the Gulf of Mexico some day. Look at the photos below to see what these watersheds look like.
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.
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.
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)
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.
Historically, Iowa was part of the Prairie Pothole Region which was full of prairie wetlands. Thousands of shallow wetlands known as potholes dominated the landscape as a result of glaciers present on the landscape. These prairie potholes and wetlands helped to store and filter water and provided excellent habitat. As agriculture and urban development have expanded, these landforms have been altered or removed, decreasing our watersheds’ ability to store and naturally treat water decreasing water quality and increasing flashy stream flow and flood potential.
How land is used may have the biggest impact on water quality and how water flows in a watershed. In places where Iowa prairie is still intact, native vegetation ensures healthy soils that can store and filter large amounts water. In densely urban areas, much of the landscape is covered with buildings, streets, and parking lots. These impervious surfaces do not allow for natural infiltration and rapidly send water to streams during rainstorms causing erosion and even flooding. In agricultural areas, compacted soils without vegetative cover easily erode into waterways and decreases soil health.
Within a watershed, any land use change has an effect. For example, if a natural wetland is removed, it must be understood the water that was once stored there has to go somewhere else. If soil is compacted, the soil loses its ability to store water causing that water to runoff somewhere else as well.
A watershed is a system of flowing water so it’s no surprise that climate and precipitation have a large impact. The amount, type, and frequency of precipitation dictates stream flow rates and how soils and vegetation will respond to rain events.
In urban and rural areas, land use changes have severely changed our landscape thus changing our watersheds’ hydrology – the movement of water. As watersheds become urbanized or soils are compacted and natural wetlands and vegetation are removed, water enters streams more rapidly. These rapid and flashy flows carry pollutants, cause erosion and even flooding in some cases. Recognizing the effects of land use change and linking that to local climate and precipitation can inform us of risks for flooding and erosion, and can help determine what actions we can take to address these problems to protect our farmland, cities, natural resources and water quality.
Working with watersheds – Using the Watershed Approach
Understanding watersheds, what defines them and impacts them at the highest level, leads to effective watershed resource management. Effective watershed resource management uses the knowledge gained from understanding watershed science including natural processes and human activity, along with social, political, economic, legal factors and concerns, and understanding resource use for agriculture, forestry, recreation, among others.
At the Polk Soil & Water Conservation District we use the watershed approach to collaborate and effectively work with partners, landowners, developers, and local leaders. Using a holistic watershed approach to water quality and soil conservation we can address resource and flood concerns, while aligning with stakeholder goals, overall improving our environment and quality of life.