The Watershed Management Plan was completed in 2016 after a comprehensive stakeholder planning process. Throughout this process, the Watershed Management Authority and its executive team routinely met to provide input and oversight and watershed residents were engaged through public meetings.
Assessment work to look at the current condition of the watershed was an important part of the planning process. This allows us to gain an understanding of the baseline health of Walnut Creek. Assessments included the following:
-239 miles of stream length assessed using desktop technologies
-In-field assessment of streams using the RASCAL method (see Chapter 5)
-Prior work detailing stream assessments through the City of Clive
-Updated flood study information
-Prior studies which referenced conditions within this watershed
Completed assessments led to valuable outcomes that will allow us to measure outcomes as we implement conservation. The following paragraphs highlight key points and conditions that were seen throughout the planning phase.
Variations in temperature and precipitation greatly influence flow patterns and pollutant loads within Walnut Creek. Rainfall has become more frequent and intense. Six of the top eight wettest years on record have occurred since 1982, while none of the driest years on record have occurred during that same period. The National Oceanic and Atmospheric Administration (NOAA) has updated its precipitation data and storm water facilities are now expected to handle runoff generated by more rainfall than expected in the past.
Flooding is a key concern within this watershed. A flood event occurred during the planning process and intensified the focus on this issue. Urban development has occurred within many flood-prone areas. Using the new NOAA data (mentioned above), this study updated hydrologic and hydraulic models as well as inundation/flood mapping for Walnut and North Walnut Creeks. The chart (right) demonstrates how much additional land is expected to fall within flood- prone areas and how much wider these flood plains are now compared to previous years.
It’s important to note flash flooding can occur outside of areas with mapped flood risk. Flash flooding can be caused by clogged inlets, storm sewers and culverts; overloaded storm sewer systems; blocked overflow paths and urban small stream flooding.
Walnut Creek is always in motion. In some areas, there is evidence of past stream meanders (curving stream segments) that were more than 500 feet from where the stream flows today. In other areas, the stream has moved several feet in only a few years. Streams are getting wider and lower. Nearly three quarters (71%) of the streams in the watershed have become incised or deeply incised—downcut over time. More than half (57%) of all field-assessed streams had moderate to severe erosion. Streams in the watershed are now 4–10x wider than they were prior to pioneer settlement. Improved stream buffers are needed. Nearly half of the smallest streams (48%) have no stream buffer or have a buffer less than 50 feet wide. Changes in land use and sources of increased sediment loads (such as cropland, gullies and construction sites with insufficient controls) can accelerate the cycle of stream evolution.
The lowest 7.6 miles of Walnut Creek are listed by the State of Iowa as an impaired waterway. E.coli bacteria are often measured at levels several times higher than water quality standards set by the State of Iowa. This poses a potential risk to health when people fish, wade, canoe or participate in other recreational activities that would put them in contact with the water.
To help this plan provide meaningful information for recommendations to the whole of the watershed, planners focused their attention on three subwatersheds, representing the primary conditions found in Walnut Creek.
By focusing scientific study on these three subwatershed types, the planners have been able to gain the most information from stream assessment field work and computer modeling. The recommendations for these three subwatersheds serve as a “template” of sorts for the balance of the watershed under similar conditions.