Two-stage Agricultural Channels in Iowa's Drained Landsape to Improve Water Quality and Long-term Resilience
Issue
Iowa’s landscape consists of a dense network of constructed drainage ditches throughout farmlands and along roadways. The trapezoidal “one-stage” design lowers water tables and efficiently routes stormflow downstream, which supports food production and ensures public safety. Two-stage channels are a modification of this design where inset floodplains are constructed with a width that is three times the baseflow channel. This design increases conveyance capacity, reduces in-channel flow velocities during storms, and decreases erosion of channel banks. During storms, suspended organic-rich sediment and attached nutrients or pollutants accumulate in the vegetated floodplain. Phosphorus deposition and enhanced microbial removal of nitrogen via denitrification in these floodplains are important but not well constrained nutrient retention pathways that our project will evaluate.
Objective
In partnership with Polk County Conservation and JEO Consulting, we will collaboratively design and assess the effectiveness of a two-stage ditch constructed to improve water quality of agricultural drainage in Drainage District (DD) 4. Our long-term goal is to inform design, construction, and maintenance of the expansive network of ditches throughout Iowa and the Midwest.
Approach
Our diverse team includes applied scientists and engineers at ISU and the University of Iowa, conservation professionals at Polk County Conservation and engineering designers at JEO Consulting. We will collaboratively design and assess the effectiveness of a two-stage channel constructed to improve water quality of agricultural drainage near Chichaqua Bottoms Wildlife area using a Before-After-Control-Impact (BACI) experimental design. In this research, we focus on the two-stage channel as a water quality practice and will measure geomorphic, water quality, vegetation and stability factors before and after construction of two-stage channel comparing them to conventional one-stage channels. Our integrated design, monitoring, and modeling approach will build on knowledge of similar practices throughout the Midwest and combine this with newly collected information to inform siting and decision-making approaches in Iowa.
Project Updates
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
October 2025
This project evaluates geomorphic, hydrologic and biogeochemical processes associated with two-stage ditch construction in agricultural drainage systems to assess the effectiveness of natural channel design in improving water quality and sediment dynamics. The experimental study design includes two control reaches representing contrasting channel conditions: a periodically dredged trapezoidal channel (Control) and a minimally maintained, naturalizing two-stage channel (Natural). These reaches represent functional end-members of agricultural drainage ditch management in Iowa. The Control site is located immediately upstream of the planned two-stage reconstruction at Chichaqua Bottoms, while the Natural site is situated in the well-studied Walnut Creek in Story County. Site selection was based on extensive field reconnaissance during the reporting period and chosen to leverage existing hydrologic and water quality datasets while controlling for variability in watershed characteristics and climatic drivers.
The two-stage ditch design was completed during in spring 2025. Subsequent coordination with Polk County Extension and JEO Consulting focused on securing permits and establishing the construction schedule. Due to permitting constraints and elevated spring flows in 2025 associated with above-average precipitation, construction has been postponed to December 2025 – March 2026. This delay provided the opportunity to obtain an additional season of pre-construction data. Since our first sampling event in November 2024, three additional rounds of sampling have been completed in March, April and June 2025. Sampling included collection and analysis of bed and floodplain sediment, as well as surface water. Intact sediment cores were collected from inset floodplains and channel beds and returned to the laboratory for measurement of denitrification rates and nutrient (NO₃⁻ and PO₄³⁻) fluxes. A stage gauge equipped with a pressure transducer was installed at the upstream control site in Chichaqua, with a barometric logger positioned by the bridge. Stage is recorded every 15 minutes, and data are downloaded approximately every two weeks. We are collaborating with other Iowa State University researchers who are collecting stage data at the Walnut Creek site. In addition, discharge measurements are being obtained using the wading method to develop stage–discharge rating curves for both the two-stage and control reaches at Chichaqua, which will support future hydrologic modeling efforts for the project. Sedimentation marker horizons were established at all three study sites, with two sedimentation tiles installed per transect across three transects. These were installed in late spring and will be sampled biannually to quantify sediment accretion rates and analyze deposited material for organic matter and total phosphorus.
Related accomplishments and activities
2 presentations. These include a presentation with partners, Polk County Conservation and JEO Consulting, at the INRC Fall Seminar Series, and Sara McMillan will teach a module on two-stage ditch design at the ISU Conservation Drainage School.
The team is working with Iowa Learning Farms to collect video footage during construction and develop a virtual field day for this project.
January 2025
The first six months of our project were focused on continued engagement and design support with Polk County Extension and JEO Consulting as we refined the design of the two-stage channel near Chichaqua Bottoms. Drainage District 44 includes multiple drainage ditches with the final section flowing into the wetlands, oxbows and relic channels of the Skunk River. The design process has been highly collaborative with quarterly meetings to seek input on design from our project team as JEO developed the design and construction documents. During this reporting period, we met in October and December 2024. To date, the design is at 90% completion with permitting scheduled for spring 2025 and construction to begin in late summer or early fall 2025. This timing is well-aligned with our research and assessment plans, allowing us to collect one full year of data before construction and continue monitoring after completion.
Our study includes two control reaches: a periodically dredged trapezoidal channel (Control) and a naturalizing two-stage channel that has had little maintenance (Natural). These two types of controls allow us to represent “end-members” of Iowa’s agricultural drainage ditches. The Control site is located immediately upstream of the two-stage ditch at Chichaqua Bottoms. Our Natural site is in the well-studied Walnut Creek Watershed in Story County. During the reporting period, we visited several candidate sites and selected these because they allow us to leverage existing data and correct for variability in climate or watershed inputs.
The first seasonal sampling in November 2024 was completed at all three sites. This included analysis of sediment (bed and floodplain) and surface water. We also collected intact sediment cores from the inset floodplains (where present) and the channel bed. These were returned to the lab to measure rates of denitrification and nutrient (NO3 and PO4) flux. Samples are still being analyzed and results will be shared during the next reporting period. Plans for the next six months include:
- Install stage gages at the Control and two-stage reaches in Chichaqua. The upstream ditch site is already instrumented through a collaborative project.
- Install sedimentation marker horizons in the floodplains (where present) in all three reaches. These will be installed prior to spring high flows and collected biannually to measure accretion rates and characterize deposited sediment for organic matter and total P.
- Analyze survey data from JEO and supplement where needed. Data are highly detailed for the two-stage section with less detail (i.e., fewer cross sections and lesser extents) for the upstream Control. There are also limited survey data available for the Walnut Creek Natural site.
- Continue quarterly experiments to measure denitrification and nutrient flux and characterize vegetation and sediment along three representative transects in each reach (Two-stage, Control, and Natural).
Other accomplishments and activities
Through this project we have strengthened collaboration with Polk County and JEO Consulting, which is highly beneficial to the design of this specific project but also will be advantageous moving forward. Our goals are to leverage this project site into a long-term monitoring location and one where we can showcase this practice as it is located on public lands. We also plan to present the study design at the Upper Midwest Stream Restoration Symposium which is held in Dubuque, Iowa, this year.