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 streams into classes by stream order using the Strahler method. The headwaters of a given stream, where perennial flow is first observed are defined as first order steams. When two first order streams meet, they join to form a second order stream. When two second order streams meet, they join to form a third order stream.
The Walnut Creek watershed includes first, second and third order streams. Most of the perennial streams in the watershed are of the first order. Lower sections of the Little Walnut, South Walnut and North Walnut Creek are second order streams. Walnut Creek is a third order stream downstream of its confluence with Little Walnut Creek.
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.
What is notable from these studies is that streambanks appear to be more unstable in urban areas. Less than 1% of streams in urban areas were categorized as “stable,” with 21% in this condition in the rural assessments. Moderate erosion is also much more noticeable in urban areas than in rural areas.
It is estimated that 11,300 tons of sediment loading per year may be caused by streambank erosion. Primary sources of this erosion are the 88.7 miles of stream segments that were categorized as “incised” or “deeply incised.” The methods of water quality modeling that generated these estimates are explained in greater detail in Chapter 6 and in the appendices of this plan.
Variety of Buffer Types
Stream conditions throughout the watershed were observed and were grouped into seven general buffer descriptions, defined on the following page. The length of each type of buffer for each stream order for the entire watershed was calculated.
Several observations can be made from this analysis.
- Nearly 50% of all “zero order” streams pass through agricultural and urban landscapes without any noticeable buffer.
- Grass buffers were most common on smaller first order streams.
- Buffers along larger streams are more likely to include overstory trees, either from historically forested areas or locations where grass buffers have been allowed to evolve into a young forest.
Current Stream Buffer Widths
Existing buffers can generally be grouped into grass and tree buffers. Knowing the width of these buffers is important in understanding how effective each buffer will be in filtering runoff and providing important habitat. Buffer width for this study is defined as the total width measured across both sides of the stream.
Where they exist, grass buffers are often wider than 50 feet. However, 33% of “zero order” streams had grass buffers that are less than this width. Combining these lengths with those sections that were observed to have no buffer at all, means that 57% of all “zero order” streams in the watershed have either no buffer or grass buffers which are less than 50 feet in width. Grass buffers can be very effective in smaller order streams in capturing sediments, reducing pollutant loads and slowing runoff velocities. Their notable absence in large portions of the watershed is a concern.
Most treed buffers exceed 100 feet in width. These buffers tend to get wider along larger streams. Many of these higher order streams pass through urban areas. Their larger drainage areas lead to wider floodplains, limiting other development opportunities. Over time, many of these open spaces have developed into wider buffers of overstory trees. These areas need to be maintained through selective clearing to prevent overgrowth or development of invasive species.