Phosphorus Contributions from Eroding Iowa Stream Banks

Date: 
Aug 2016

Issue

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.

Objective

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

Approach

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.

Project Updates

July 2020

FINAL REPORT: 

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.

Publications:

  • Williams, F., P.L. Moore, T. Isenhart, and M.D. Tomer, 2020, Automated measurement of eroding bank volume from high-resolution aerial imagery and terrain analysis, Geomorphology, v. 367, 107313.
  • Williams, F., 2019, Combining field and automated methods to estimate bank erosion: a regional estimation of sediment and Phosphorus loads. M.S. thesis, Iowa State University.

Other accomplishments:

Category: