KEY CONCEPTS 1. The streams of this watershed currently are not “natural”
Most of the streams within the watershed have been altered by human activity. Most streams that were present naturally have been straightened and widened. Other streams have been created by grading or tiling. Some streams which did exist have been enclosed within pipes and culverts.
2. Stream assessments identify problems
Nearly half of the significant streams in this watershed were reviewed in the field. Of these, 57% had signs of moderate to severe streambank erosion. Only 1% of streams in urban areas were considered to be stable.
3. Increases in flow make small streams act like larger rivers
Streams throughout the watershed are often wider and lower than they were historically. They have formed wider and deeper channels to convey larger volumes of water.
4. Buffers wanted
Buffer strips along streams were either absent or were not wide enough along almost half of the smaller streams across the watershed. This means nearly 100 miles of stream length could use better buffers.
HOW DO THESE CONCEPTS INFLUENCE DEVELOPMENT OF THE PLAN? Current levels of streambank erosion and management are far above historic levels. Private property and infrastructure near streams are put at great risk. Adequate stream buffers can reduce runoff, slow velocities, resist erosion, filter pollutants and provide important habitat.
Stream order is an important concept to understand in watershed planning. Streams of different sizes often have different challenges and opportunities for improvements. The IDNR has created maps of streams throughout the state. They group these example, at the confluence of a first and second order stream, the downstream segment remains classified as a second order stream. The second order stream does not become a third order stream, until it meets another second order stream. (1) streams into classes by stream order using the Strahler method. The headwaters
The Walnut Creek watershed includes first, second and third order streams. Most of a given stream, where perennial flow is first observed are defined as first order of the perennial streams in the watershed are of the first order. Lower sections of steams. When two first order streams meet, they join to form a second order stream. the Little Walnut, South Walnut and North Walnut Creek are second order streams.
When two second order streams meet, they join to form a third order stream. Two Walnut Creek is a third order stream downstream of its confluence with Little Walnut Creek.
Throughout the watershed, there are many swales, ditches, depressions and small streams that drain significant areas, yet do not have perennial flow and are not classified as first order streams. These drainage paths are clearly visible on LiDAR topographic imaging available through the State of Iowa. To better understand the properties of key flow paths throughout the watershed, many of these features have been mapped as “zero order” streams. As part of this effort, they were studied in similar detail to the more defined stream segments within the watershed. Most of the streams reviewed in this plan are "zero order" and "first order" streams.
Horizontal Stream Character
These basic categories were used to describe the general types of stream alignments that have been observed during field assessments and desktop GIS information review. They are used to evaluate the relationship of a stream to the surrounding areas and the potential for movement of the stream.
- Horizontal Stream Characteristics
- Character Type Description
- B Braided
This is where the stream is divided into multiple paths, separated by islands or where multiple points of erosion have occurred.
Meandering The stream weaves back and forth through a series of tight curves.
These are remnants of past channel locations. As the stream moves and meanders, low spots are left in the landscape where the stream channel used to be. They are often filled with sediment during large flood events, becoming much more subtle features over time. Since these are no longer the primary path of flow of the stream, they were catalogued as “zero order” streams, even though they usually were found alongside higher order streams.
These are segments where a sudden drop or waterfall occurs. Below the drop, the stream is actively incising (downcutting). These knickpoints tend to work their way upstream over time, as erosion grows more significant. The downstream channel is left wider and flatter than before, able to convey larger flows at lower velocities than the stream above. These are usually located on smaller order streams. Often, the downstream alignment follows a straightened path. The knickpoint is usually at a defined point, but for the purpose of this analysis, longer segments were defined as this feature type when the knickpoint existed within them.
Almost all segments have been altered, straightened or restricted from movement in the past. For this analysis, the stream was defined as straightened if it was a stream with constant flow where the alignment and general character of the original drainage ditch construction was clearly evident.
N/A No Definition
The location and character of most of these segments have been significantly altered by human activity, but they don’t clearly fall within any of the definitions above.
Streambank Stability Analyses
Two separate stability studies were reviewed as part of development of this plan. The first was completed within the scope of this planning process by staff from the Polk County Soil and Water Conservation District. Their efforts completed a RASCAL (Rapid Assessment of Stream Conditions Along Length) survey of more than 28 miles of streams, primarily in the rural areas of this watershed. This assessment was completed in the field during the summer months of 2015, walking along each segment using a GPS data collector equipped with the RASCAL software to gather information about a variety of characteristics of the stream.
The second was the 2014 Clive Stream Assessment Report Update. This study reviewed the stability conditions of streambanks of more than 13 miles of streams within the City of Clive. Most of these assessments were completed within the publicly owned Clive Greenbelt. This report was an update to assessments completed in 2009, which was conducted using the RASCAL protocol.
Collectively, these two studies evaluated streambank stability for nearly 42 stream miles within this watershed. This represents a current evaluation of 44% of the total length of first through third order streams in this area. Other studies have also been completed in the past by other cities, which were reviewed as part of development of this plan. In some cases, specific GIS data from these other studies was not available for analysis. In other cases, the data provided was from before 2014, so it was not considered a current evaluation of stream conditions and was not included in the statistical analysis for this plan.
One method of stream path change involves sediment settling out of fiow in areas where velocity is reduced (i.e. inside curve of stream bends, downstream of obstructions like downed trees or debris piles). As sediment collects, it piles up and can change the normal path of fiow, often forcing it toward the outer banks of the stream. As this happens, stream velocity increases along the edges of the stream, leading to more streambank erosion. The sediment lost from the streambank is carried downstream until it reaches a place where velocities slow to a point where sediment can settle out. This is a cycle of erosion and deposition which repeats over time. The stream meanders, and fiow paths form and disappear. In some areas along Walnut Creek, there is evidence of past meanders that are more than 500 feet from the current stream.
Changes in land use can magnify this effect to levels not seen within the native landscape. Increases in the rate and volume of runoff result in faster and deeper streamflows. These effects increase the erosive force on the bed and banks of the stream. This shear force cuts against stream banks, widening the stream. The bed of the stream begins to be incised, or downcut.
Sources of increased sediment loads, such as cropland, gullies and construction sites with insufficient controls can accelerate the cycle of stream evolution. Over a long period of time, a wider and flatter stream is created, capable of conveying higher flow volumes. The new channel is often several feet lower than the natural stream bed. This results in a stream that is disconnected from the flood plain above. When streams are disconnected from their floodplain, it is more difficult for water to spread out across the floodplain where it can flow more slowly allowing for absorption and filtration. Disconnection also changes the habitat conditions for a variety of plants, insects and animals which rely on having access to a stable boundary between stream and floodplain.
Streams throughout the watershed have changed significantly since pioneer settlement. The early surveyors measured the width of streams as they surveyed each section line (or the edge of each square mile of land). They would measure the width of streams by the numbers of links on their surveyor chains. Each chain had 100 links and the chain was 66 feet long. So each link in the chain was eight inches in length. By reviewing the original survey maps of this area, we can determine the width of the stream in their time (mid 1800s) and compare it today’s conditions.
Two conclusions can be reached:
1. Fewer streams existed prior to settlement. The surveyors recorded streams as small as one foot in width. Many of the “zero” and some of the first order streams that exist today were not drawn on their survey maps and no measurement for stream width was recorded. As agricultural and urban uses have increased the portion of precipitation that is converted to surface runoff, new streams have been created. New streams were also created by draining the landscape to support agriculture during the late 1800's and early 1900's through installation of tiles and ditches.
2. The streams that did exist prior to settlement were much narrower than those we see today. The table on the following page notes changes in stream width between what was recorded by the original surveyors and what can be measured from the LiDAR survey of the state that was completed in 2007-2008.