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KEY CONCEPTS

What is the watershed? The Walnut Creek Watershed is the 83-square-mile area in Central Iowa that drains toward Walnut Creek either by direct flow or through tributary streams, ditches, subsurface tiles and storm sewers.


Soil characteristics The properties of soils within the watershed influence how much rainfall is absorbed by the landscape and how much direct stormwater runoff is created during rain events.


Terrain and topography Areas with steeper slope may be more prone to erosion, instability and often will direct surface runoff more quickly toward receiving streams.


Land use changes This watershed is one of the most rapidly developing in the state. Currently urban and rural land uses are nearly evenly split. In a recent ten-year period, over 4,200 acres (6.6 square miles) was developed into suburban landscapes.


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HOW DO THESE CONCEPTS INFLUENCE DEVELOPMENT OF THE PLAN?

The characteristics of this watershed have changed significantly over time. Reviewing past, present and expected future conditions allows critical areas and influential properties to be identified. Such factors affect stormwater runoff patterns, are indicators of potential pollution sources and can highlight other factors which could have a negative impact on water quality or stream stability.

Location and Geography

A watershed is an area of land that drains to a common point. The Walnut Creek watershed covers approximately 53,000 acres (82.8 square miles) across eastern Dallas and western Polk Counties. Its most western source falls within the community of Dallas Center. The footprint of the watershed extends across eight communities and other unincorporated areas within each county. Walnut Creek drains generally from northwest to southeast, meeting the Raccoon River northwest of Water Works Park in Des Moines, upstream of the water intake to the Des Moines Water Works plant.


The Raccoon River drains into the Des Moines River in downtown Des Moines. The Des Moines River then flows generally southeast, first through Red Rock Lake in Marion County, then on to the Mississippi River at Keokuk. The Mississippi River flows south, ultimately reaching the Gulf of Mexico in Louisiana.

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The Tributaries of Walnut Creek

Dozens of smaller streams and three major tributaries feed into Walnut Creek. North Walnut Creek runs generally from north to south, draining parts of Grimes, Johnston, Urbandale, Des Moines, Clive and Windsor Heights. South Walnut Creek is more commonly known as the stream which passes through Country Club Lake in Clive. This stream drains primarily southwest to northeast, collecting runoff from portions of West Des Moines and Waukee in addition to Clive. Little Walnut Creek generally flows from west to east, beginning in rural Dallas County and flowing through rapidly developing portions of Waukee, Clive and Urbandale.


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Topography and Terrain

There are two distinct types of land forms within this watershed. The upland parts of the watershed are generally fiat, featuring meandering flow paths and low spots. These features are what remains of the prairie pothole wetlands formed by the Des Moines Lobe of the Wisconsin Glacier (1). These pothole wetlands were drained after pioneer settlement to improve agricultural production. This was accomplished through installation of subsurface tile drains and engineered ditches during the last half of the 1800s and the early 1900s. Many of the smaller streams which exist today did not exist before this landscape was altered.


The topography in the lower parts of the watershed becomes steeper, with many more hills and valleys. The surface of these areas was shaped by large scale erosion caused by the melting of the Wisconsin glacier. Surface slopes in excess of 5% are typical, with slopes greater than 9% scattered throughout this part of the watershed. These areas are also within the historical footprint of the Des Moines Lobe. Its southern edge fell just north of where Walnut Creek flows into the Raccoon River.


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Soils

There are many properties of soils that have a significant impact on the quality and quantity of stormwater runoff. Three of the key properties are hydrologic soil group, hydric conditions and soil erodibility.


Hydrologic Soil Group

The ability of water to move into and through the ground is different with every soil. Soils with more clay tend to be less permeable. Very little water can enter surface of a clayey soil, and the water that does enter takes a long time to move through that soil. In contrast, sandy soils allow water to enter and move very freely. Soil properties like these influence how much surface runoff will be generated from open spaces when it rains. Different soils are classified into different hydrologic soil groups, on a scale from “A” to “D.” Group “A” soils allow water to infiltrate into the soil and percolate through the soil very easily. These soils have a higher sand content and will absorb more rainfall, causing less runoff to be developed. Group “D” soils include clays and other soils that inhibit the free movement of water.


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Soil Compaction/Topsoil Removal

County soil maps typically designate soil groups for rural landscapes. When land uses change from rural to urban, the ability to infiltrate and store water by the soil is impacted by the removal of topsoil materials and compaction of subsoils by heavy equipment. This typically results in additional stormwater runoff after development, unless techniques are applied which restore these soil functions.


Hydric Soils

The presence of hydric soils indicates wetlands are present or were in the past. Finding these soils highlights opportunities to protect or restore wetland features. Wetlands capture and filter runoff, improving water quality and reducing its volume.

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Soil Erosion Prediction Factors

The erosion potential for soils has been classified on county soil maps. These maps identify a number, or coefficient, which can be input into the Revised Universal Soil Loss Equation (RUSLE). This equation was developed by the USDA to predict erosion rates from landscapes (not including streambank or gully erosion). This coefficient changes with soil type and surface slope. This coefficient is a decimal value between 0 and 1. It is one factor that predicts how much soil would be expected to be washed off the surface of a given area. The RUSLE formula also includes factors that vary based on the length and steepness of a given slope, and other surface conditions.


Slopes

The change in surface elevation over a given length has many impacts. Longer and steeper slopes have greater potential for erosion. Steeper slopes also lead to faster runoff velocities. This decreases the time it takes for water to reach a receiving stream, resulting in a larger pulse of water (or peak flow rate) reaching a given point.


As runoff rates increase, steeper streams may be vulnerable to incision, or downcutting of the stream bottom. As downcutting continues, the stream bottom will flatten and sideslopes will cave in, widening the stream until a point is reached where the stream can convey the larger runoff volumes and rates at a velocity that causes less erosion. Surrounding steep slope areas can also become unstable, sloughing or sliding—especially during extended periods of wet weather. (Refer to Chapter 5 of the plan for details about streambank stability.)


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The Native Landscape

The Walnut Creek watershed was a much different place when initial land surveys were performed in the mid-1800s. Much of the landscape was covered by tallgrass prairie. Grasses and wildflowers, reaching eight feet in height, would have stretched to the horizon in every direction. The deep roots of these plants (some up to 15 feet below the surface) combined with worms and burrowing animals to create several feet of the loose, fertile, porous black topsoil that Iowa is known for. These soils allowed nearly all of the rainfall that fell on the landscape to soak into the ground through infiltration. Most streams were formed from natural groundwater outflows or springs. Prairie lands were kept largely clear of trees and shrubs by regularly occurring grass fires, limiting the opportunity for less fire tolerant species to flourish.


The prairie pothole landscape featured a largely flat surface with depressed areas. These pothole areas usually featured wetlands fed by natural springs. Runoff from very large storms would collect in these areas until it either evaporated, infiltrated or overflowed into an adjacent depressed area or receiving stream.


Savannas would have covered the remainder of the landscape, usually along the larger streams and hills in the southeastern part of the watershed. These savannas would have also been quite different from the woodlands that are familiar to us today. Savannas lacked much of the understory brush and invasive species that currently make many of our forested lands difficult to walk through. Many of the current invasive species did not arrive within the watershed until the early 20th century, brought over from Europe and Asia. The lack of understory growth allowed more sunlight in to the floor of the forest, supporting shade tolerant native plants. The presence of these deeper rooted plants would have made the surface below the canopy significantly more resistant to erosion than what exists today.


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Pioneer Settlement and Agriculture

The deep, fertile soils created by the prairie supported agricultural development. During the late 1800s and early 1900s, drainage projects installed tiles and ditches allowing the pothole wetlands to be dried out and farmed. The draining of wetlands, removal of vegetation and soil compaction by use of heavy farm equipment reduced infiltration and increased runoff volume. As a result, a larger portion of rainfall was transported to streams by surface or tile flow. Installation of tiles, culverts and ditches allow this larger volume of water to flow downstream more quickly. Water was allowed to rush downstream, with larger portions arriving at given points downstream at nearly the same time. Shortening the travel time for water flow allows a larger portion of the runoff to arrive at a given point at nearly the same time, making the rate of runoff increase even more dramatically than the runoff volume.


Today, virtually all native prairie is gone from the watershed. Grass swales and buffers need maintenance through controlled burns, mowing or grazing to keep out tree growth. Without this maintenance, forested areas along waterways and ravines have become crowded with invasive species and underbrush. This shades out the native erosion-resistant ground cover. Many of these areas have experienced significant soil erosion from surfaces and stream channels.


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Recent Land Use Change

This watershed includes some of the most rapidly growing urban areas in the state. Aerial photos from the 1930's and 1950's show a watershed with limited urban growth primarily within the far eastern (downstream) parts of the watershed. Shortly thereafter, development of the interstate system including I-235 and I-35/80 facilitated westward expansion of the metro area.


During the 1990's urban growth began to significantly push past the I-35/80 corridor. Over the following decades, the communities of West Des Moines, Clive, Urbandale and Waukee saw significant growth in the western part of the watershed. More recently, the communities of Grimes and Johnston have seen more rapid growth in the North Walnut Creek basin.


During the period between 2001 and 2011, eight percent of the watershed developed into urban land uses. This included nearly 4,300 acres of land, or about 6.7 square miles. This period included both times of intense economic growth as well as the 2–3–year recession period late in the first decade of the 21st century. As of 2011, urban land uses covered 43% of the Walnut Creek watershed.


Between 2001 and 2011, 8.1% of the watershed developed into urban uses. The landscape is nearly evenly split between cropland and urban areas.

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Six of the top eight wettest years on record in the watershed have occurred since 1982