Watershed-scale Phosphorus Inputs from Streambanks

Aug 2019


Streambanks were identified in the Iowa Nutrient Reduction Strategy (INRS) as a potentially important but poorly quantified source of phosphorus (P) to surface waters. New research from Iowa and other Midwestern states has quantified streambank contributions, with results varying widely. Most of these efforts have focused on a small number of eroding banks in relatively small watersheds. Few of these studies have extended for more than five years and may therefore not be representative of the full range of inter-annual variability in hydrology and P contributions. Recent projects funded by INRC and other sources have supported long-term monitoring of the extent and magnitude of eroding banks on Onion Creek in central Iowa, which has been identified as a potentially large contributor to the sediment and P loads of Squaw Creek. Results from the Onion Creek studies found major variation over time, highlighting the importance of long-term monitoring to understand the causes of variability in P sources, dominant P forms and rates of P export. A related INRC-funded effort that is nearing completion addresses the need to make estimates of streambank-derived P scalable to larger areas of the state, as the field-intensive methods that are most often used for quantifying bank erosion rates and bank-derived P are impractical in watersheds much larger than 100 square kilometers. Methods that make use of high-resolution geospatial data are more efficient at this scale. A widely-used technique at this scale is to digitize streambanks from aerial photos in sequential years to estimate bank migration rates. When combined with high resolution digital elevation data or a sample of bank heights from field measurements and estimates of P concentrations in bank materials, P contributions can be estimated on these larger scales. This approach still takes significant time and involves some subjectivity in bank delineation.

To address these limitations, we have developed the Aerial Imagery Migration Model (AIMM), which uses a band index from high-resolution multispectral aerial imagery to automate detection of streambanks and acquires bank heights from LiDAR-derived elevation data. In initial testing, AIMM compares favorably with manual bank digitization in accuracy, is extremely efficient and eliminates the subjectivity of manual methods. However, testing of AIMM has been very limited. Exploration of the potential of AIMM to address additional reach- or basin-scale management problems also remains to be addressed.


This research will extend recent work identified above to explore questions related to the use of monitoring data to evaluate practice effectiveness. Since AIMM enables efficient detection of bank erosion and deposition over large areas, it has the potential to provide valuable insights into the spatial distribution of problem areas and to improve management decisions targeting areas of greater erosion risk.

The first objective is to continue and extend ongoing work in the Onion Creek watershed that began in 2011. Periodic eroding-length assessments and erosion pin measurements will continue along with continued manual and automated water sampling near the watershed outlet. The collection of additional data will enrich the existing dataset and address new questions that leverage the diversity of hydrological conditions encountered in the long-term study.  

The second objective is to extend the application of AIMM beyond the initial test watersheds to the entire state. In this effort, we will compare AIMM estimates of erosion and deposition to those obtained by the Iowa DNR using manual bank delineations in a subset of watersheds around Iowa. This comparison will allow a test of the sensitivity of AIMM to variables that differ among landform regions. AIMM will then be used in combination with soil P-concentrations estimated from GIS layers to estimate P contributions from streambanks for each HUC-10 watershed in the state. In select sites where the data is available, these estimates will be compared with annual total P export derived from continuous monitoring at gaging stations.

A third objective of this research is to use AIMM and manual bank delineation to evaluate the effectiveness of bank stabilization and flow-training structures in reducing streambank contributions of P at a multi-reach scale.


Field measurements in Onion Creek will continue to use the same methods established in previous work. A network of banks selected randomly from a complete set of eroding banks identified in a full watershed survey has been set up for erosion pin measurements at horizontal intervals along the bank. Water samples will be collected, sediment concentration determined by filtration and dissolved and total P concentrations determined by photometric and persulfate digestion methods, respectively.

For Objective 3, research will focus on a portion of southwest Iowa’s West Nishnabotna River, where eight streambank stabilization projects, totaling approximately 6,000 linear feet, have been completed since 2012 on a 16-mile stretch. We will use AIMM to estimate erosion and deposition in this reach, compared with 16-mile reaches upstream and downstream that have had comparatively little streambank stabilization. Geospatial analysis for the second and third objectives will be performed in the Applied Geomorphology lab at Iowa State University. Bank erosion estimates from manual digitization will employ an existing dataset available from partners at the Iowa DNR that is coincident with the 2007-2010 LiDAR data. Additional digitization will be completed for a second, more recent year where high resolution imagery is available.

Results of this research will be combined with those of a collaborative INRC-proposed project, Linking Agricultural Practices to Water Quality Improvement: The Importance of Scale in Accurately Characterizing N and P Loads Delivered to Iowa Streams (Crumpton et al.). The combined dataset will provide insight as to the effect of watershed scale on the relative contribution of upland versus in-stream sources of P.

Results of this research will be disseminated in multiple forms, including for scientific journals and Extension publications and presentations at appropriate regional or national academic conferences and workshops for watershed managers and engineers. The team also plans to make AIMM and supporting documentation available as an open-source ArcPy script and an ArcGIS plugin.

Project Updates

Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.

December 2023


Key Research Questions:

The primary aim of the project was to use a remote sensing toolbox (AIMM, developed with support of an earlier INRC award) to efficiently estimate inputs of P from streambanks across large watersheds. A secondary aim was to continue long-term monitoring and data analysis from eroding streambanks in Onion Creek near Ames.

Research Findings:

Our primary finding in our focus watershed (Nishnabotna River, IA) was that streambank erosion was potentially responsible for a large proportion of the sediment and P inputs to the river from 2009 to 2018, according to AIMM estimates. The most important reason that riverbanks contributed net sediment and P was that the Nishnabotna River appears to be undergoing widespread floodplain shaving, which occurs when the depositional surfaces within a meander belt are at a lower elevation than the eroding cutbanks. The Onion Creek study compiled 10 years of erosion pin data and showed that stream reaches with the greatest unit stream power appeared to produce the most bank erosion, and that most of the erosion was recorded after large runoff events.

Related activities and accomplishments:

- 1 workshop

- 4 presentations

Publications / Journal Articles:

Williams, F.F., P.L. Moore, J.BV. Allen, T. Isenhart, J. Thomas, J. Kovar, and K.E. Schilling, 2023. Sediment and phosphorus contributions from eroding banks in a large intensively managed watershed in western Iowa, United States, Journal of the American Water Resources Association, https://doi.org/10.1111/1752-1688.13164 Allen, J.V., Streambank erosion over time and space, M.S. thesis, Iowa State University.

Leveraged Dollars:

Two directly-related proposals were declined ($212,009 total requested from USGS and INRC), two directly related proposals were funded ($32,663 from INRC, $17,776 from Leopold Center), and one additional more distantly-related proposal was funded by the INRC ($144,886).

Other Accomplishments:

This study provided information important to the mission of Hungry Canyons Alliance, which helps counties and landowners address erosion problems in their rivers and streams. As a consequence, we presented work-in-progress results of this study to the HCA stakeholders group on three different occasions over the period of this award.



June 2023

The first half of 2023 consisted primarily of three activities. 1) periodic monitoring of the erosion pin network in Onion Creek; 2) continued work with application of AIMM to new watersheds; and 3) completing revisions to the manuscript that is the centerpiece of this project, which summarizes the application of AIMM to the Nishnabotna River watershed in southwest Iowa. Jade Allen, a M.S. student who contributed to this project, successfully defended her thesis in December, but graduated this spring.

Project Activities:

- 3 presntations.

- 1 workshop.

Publications/ Journal Articles:

One new article has been accepted for publication in the Journal of the American Water Resources Association:

Williams et al., 2023, Sediment and phosphorus contributions from eroding banks in a large intensively-managed watershed in western Iowa, USA.

Proposals submitted:

One proposal was submitted to the INRC for follow-up work that would effectively complete all key terms in the sediment and P budget within a portion of the Nishnabotna River corridor - $104,963

Total cumulative dollars - $ 407,334 (five proposals that build on this project have been submitted).

Other Ativities:

August 12 outreach activity associated with the annual meeting of the National Association of County Agricultural Agents to be held at Whiterock Conservancy. PI will co-lead a discussion of bank erosion and its impacts on water quality.

December 2022

From July until Dec. 2022, most of the focus has been on fulfilling Objectives #1 and #3 from the proposal: 1) continuing long-term monitoring of Onion Creek erosion pins and synthesizing all data to date; and 3) examining the impacts of bank stabilization structures on bend- and reach-scale channel migration and P storage. The graduate student, Jade Allen, whose primary responsibility was to undertake this work, is defending her thesis in Dec. 2022, which was focused on these objectives. The findings will be formatted for submission to peer-reviewed journals.

On Objective 2 (scaling up AIMM analysis from small watersheds to larger areas), we have encountered major computing barriers due to the large amount of data required. We have been exploring options for server-side analysis using ArcGIS online, with the assistance of the GIS Facility and ESRI.

January 2022

Research has progressed on two of our three objectives since the last report: First, while regular monitoring of our erosion pin network continues on Onion Creek,a graduate student focused much of her effort this period on compilation and analysis of the data accumulated in the past 10 years at Onion Creek. This included developing a robust stage-discharge relationship for Iowa Flood Center’s bridge sensor ONIONCR01, as well as developing a framework for ensemble analysis of erosion pin data from three different generations of pins in Onion Creek. Another graduate student installed a flow and water quality monitoring station near the mouth of Onion Creek as part of a separate project, but with the intent of providing data on discharge and nutrient export that can benefit this project.

Development and customization of AIMM continues (the ArcPro package that processes aerial imagery and high-resolution DEMs to estimate channel migration rates and resulting net mass loads of sediment and P related to bank erosion in river systems). The original and fully-functional code and ArcPro toolbox is still maintained and updated on the AIMM creator’s GitHub website (https://github.com/forrestfwilliams/AIMM_Stable), along with detailed documentation. Some additional scripts have been developed intended to allow AIMM to be extended from a diagnostic tool to one that can identify probable future rates of bank erosion. These scripts resolve the position of the channel centerline compute channel curvature, a key variable that influences bank erosion rates. This quantity will allow us to ensure that comparisons between erosion rates on different bends account for differences in curvature between bends.

Other activities included one presentation by graduate student Jade Allen on some of her experiences studying bank erosion at INRC’s “Ag in the Classroom” event in August. We have also combined some of the methods developed or used for this and previous INRC projects (bank sediment sampling, Total P extraction from sediment, AIMM channel migration analysis) to assist the City of Ames with an assessment of the nutrient-reduction value of stream and riparian restoration in the Ioway Creek-Duff Avenue corridor. A report will be submitted to the City in mid January with two students as lead authors.

July 2021

During Jan. to June 2021, research progressed on three fronts.

First, refinement of the AIMM (Aerial Imagery Migration Model) workflow has streamlined analysis and implementation. In addition, the code has been repackaged as an ArcPro toolbox rather than a collection of independent ArcPy scripts. The code is maintained and updated on the AIMM creator’s GitHub website (https://github.com/forrestfwilliams/AIMM_Stable), along with newly developed documentation. New graduate student Jade Allen is becoming acquainted with the workflow while also testing methods for efficient image mosaicking to streamline the use of AIMM over large geographic areas.

Initial information has also been collected for the bank stabilization study reaches in the Nishnabotna River watershed. Information required for bank stabilization structures includes structure location and type, date of installation and any dates of known maintenance. This information will be checked against direct and remote observations prior to analyzing meander migration patterns from AIMM analysis of the study reaches.

Finally, significant progress has been made in the Onion Creek study, where bank erosion continues to be monitored at least quarterly using our erosion pin network. The data collected there since 2011 has been organized and compiled to allow a full longer-term analysis of spatial and temporal patterns in bank erosion within the watershed. This rich dataset has been used as a basis to attempt a regional calibration of the NRCS erosion severity scale, a qualitative assessment tool that is widely used to estimate sediment and contaminant inputs to streams, but which currently has uncertain regional relevance.

December 2020

A new graduate student was recruited and one left the project. With the help of the students, research began to test the BEHI assessment protocol in Onion Creek to create a calibration curve for central Iowa streams.

Outreach included a presentation at the Hungry Canyons Alliance fall meeting in August.

December 2019

Periodic field measurements of erosion pins in Onion Creek are being maintained with two undergraduate technicians. A new graduate student is being recruited to lead the research outlined for this award.

Two presentations are planned for the first half of 2020, the Upper Midwest Stream Restoration Symposium, Stillwater, Minnesota, in February, and the Hungry Canyons Alliance annual stakeholder meeting in March.