Phosphorus Contributions from Eroding Iowa Stream Banks
Stream bed and bank erosion represents a significant contribution to nutrient export from Iowa’s streams, exacerbating downstream water quality concerns. Yet the size of this contribution is not well known. While in-field and edge-of-field practices for nutrient reduction have shown promise for reducing nutrient loads in streams, these have little or no impact on nutrients derived from within the stream corridors.
This research will extend and build on recent work in the Onion Creek watershed, plus establish new measurements in the loess-dominated ecoregion in southwestern Iowa. The goal in both sites is to quantify the contributions of stream bank erosion to watershed phosphorus (P) export, and develop new measurement methods that may enable more efficient, widespread assessment of the role of stream bank erosion in watershed nutrient budgets
Onion Creek work will be extended using the new permanent hydrological and water-quality monitoring station near the mouth of the creek, installed in fall 2015. The same types of measurements also will take place in the Nishnabotna River watershed in southwest Iowa. Periodic manual sampling of stream water will be done to establish the turbidity/P relationship. Soil samples will be taken from each major unit represented in eroding stream banks, and will be analyzed for P concentration and bulk density. Erosion rates will be estimated from aerial-photo analysis and converted to P source estimates.
In a small local watershed (Onion Creek), researchers continued to find eroding streambanks to be a substantial but strongly variable source of phosphorus (P) to streams. Inter-annual variability in hydrology appears to control the downstream export of P, but preparation of bank materials for erosion remains an important process in low-flow periods.
Work in the larger Nishnabotna River watershed led to the development of an automated method and GIS tool to detect changes in the position of streams over the period between two high-resolution aerial photos. Combined with high-resolution elevation data from LiDAR, this method allowed automated estimation of P contributions from streambank erosion in all third to sixth order reaches of the Nishnabotna.
The method was tested against alternative methods in a trial watershed (the South Fork of the Iowa River) and provided results comparable to existing, but time-consuming methods. The limitations of image resolution make use of this method in headwater streams imprecise, but comparison of bank erosion rates by order indicates that the volume of sediment (and likely P) introduced into the channel by bank erosion increases almost exponentially with stream order. Final analysis of total P from soil cores taken in the Nishnabotna watershed is currently taking place, so current estimates of P inputs in that system are preliminary. A new INRC-funded project will begin shortly, and will address in part the questions raised in this project.
This research resulted in the creation of a new GIS tool called AIMM, which will be made available through GitHub and through the PI's research website.
Monthly and event-based bank-erosion monitoring and periodic water sampling continues within the Onion Creek watershed. Soil samples acquired from the Nishnabotna field sites are being analyzed for different fractions of phosphorus (total, soluble reactive, etc.) in Dr. John Kovar’s lab at the USDA Agricultural Research Service National Lab for Agriculture and the Environment (NLAE). Student employees have been digitizing streambanks in the Nishnabotna watershed for the final phase of the analysis, wherein the volume of soil and P entering surface waters via bank erosion are estimated. The graduate student working on this project has also adapted a remote sensing method to detect change in stream channel position over time and is testing this method for potential broad-scale application to efficiently map zones of bank erosion.
Routine work continues in the Onion Creek watershed to gather water chemistry data and periodic streambank erosion information. At the end of December 2018, lab work at the NLAE on soil sampled from the Nishnabotna watershed was on hold due to government shutdown, but the samples are in the queue for winter processing. A watershed-scale method for automated detection of bank erosion has been developed and testing is underway to compare results from the method with other frequently used methods of bank erosion estimation.
Monthly and event-based bank-erosion monitoring and periodic water sampling continues within the Onion Creek watershed. Soil cores collected in fall 2017 from streambanks in the Nishnabotna watershed were described and sampled for laboratory analysis of soil texture, bulk density and geochemistry, including phosphorus. The bulk density measurements are complete, and the other lab analyses are underway. Project personnel met in June to discuss plans to complete the project during a third year with no additional costs.
Monthly bank-erosion monitoring and periodic water sampling continue within the Onion Creek watershed. Eighteen deep soil cores from streambanks in the Nishnabotna watershed collected in late 2017 are being described and sampled for laboratory analysis of soil texture, bulk density and geochemistry, including phosphorus. A geospatial algorithm also is being developed to more efficiently identify and map eroding streambanks using only digital elevation data and stream centerlines. This tool would expedite and remove subjectivity from watershed-wide estimation of bank erosion.
Monthly bank-erosion monitoring and periodic water sampling continued within the Onion Creek watershed. Late 2017 saw significant progress in new work in the Nishnabotna River watershed, where four of the team’s collaborating researchers identified, secured permissions and obtained deep soil cores from 18 stream banks within the watershed. These cores will be described and analyzed for bulk density and several geochemical properties (including two forms of P) in early 2018, and the results will be used along with remotely estimated bank erosion rates to estimate bank-derived P inputs from a subset of stream reaches. These will be scaled up to the full watershed using spatial statistical data developed with collaborators from the Iowa DNR.
A water sampling station near the mouth of Onion Creek was repositioned and upgraded. Preliminary tests were completed for a new methodology of bank erosion monitoring. It is expected this structure-from-motion photogrammetry can be used at multiple sites in the Nishnabotna and Onion Creek watersheds. Prior Onion Creek datasets for ensemble analysis were reformatted with new data. Nine preliminary samples of Nishnabotna floodplain alluvium from diverse land uses were acquired to assess the need for land-use sampling stratification. Phosphorus concentrations did not vary systematically across three land uses, so sampling can proceed in buffers without fear of bias.
A watershed-wide eroding bank re-survey for Onion Creek is complete — the previous survey had been completed more than six years ago. A new erosion pin network was established in a subset of the identified eroding banks, which will allow continued monitoring of sediment and soil phosphorus for years ahead. A preliminary analysis of GIS data for the Nishnabotna watershed was done, in addition to the initial field-site selection process for obtaining core samples and bank erosion rates.
Work resumed in the Onion Creek watershed with plans to redo an earlier watershed-wide eroding bank survey done there. A field-planning meeting was held in the Nishnabotna watershed. Key geospatial data has been obtained, and both the field-site selection process and aerial-photo analysis for estimating bank erosion rates on the Nishnabotna are underway.
A graduate research assistant was hired to manage and conduct this research project, beginning in January 2017. The process began to obtain matching funds from the Hungry Canyons Alliance.