Phosphorus Transport in Iowa Streams: Importance of Stream Bed and Bank Erosion

Date: 
Feb 2013

Issue

There is a growing body of evidence that much of the sediment and P delivered to surface waters from agricultural landscapes originates from stream bed and bank erosion. The need to better understand this topic of legacy phosphorus was identified as a research need in the Iowa Nutrient Reduction Strategy and highlighted in comments about the strategy.

Objective

This project, initiated in 2011, is conducted within the Onion Creek Watershed, a tributary to Squaw Creek in Boone and Story counties. Specific objectives are to measure watershed sediment flux; estimate sediment (and P) from stream bank erosion, gully erosion, sheet and rill erosion and any other sources in the watershed; and evaluate the effectiveness of conservation practices established as components of the Onion Creek Watershed Project in reducing sediment flux.

Approach

A permanent water quality sampling site, equipped with a portable sampling unit, has been established near the outlet of the Onion Creek Watershed. A detailed field survey of riparian land use and severely eroding banks was conducted in 2011. Stream bank recession rates are estimated using erosion pins installed in a randomly selected subset of severely eroding banks along reaches within each land use classification. Total P loss from stream banks is estimated by combining total soil loss with average stream bank soil P concentration. Phosphorus adsorption/desorption studies on intact cores will be related to the key sediment characteristics that are likely to control P retention and release.

Project Updates

March 2016

The first quarter of 2016 was marked by sustained base flow in Onion Creek due to the warm, wet winter. Ice-out occurred during the third week of February as a result of snow melt over this period. Water samples were collected on a routine basis during baseflow conditions and at multiple times throughout the duration of storm flows. Stream bank erosion pins were measured at six intensively monitored sites where snow conditions allowed. Data are being analyzed and will be incorporated into the now four-year recording period. Brian Noonan, the graduate student supported by this project, successfully defended his M.S. thesis. Noonan’s research supports findings in Iowa and surrounding states that much of the sediment and phosphorus delivered to streams from agricultural landscapes can be attributed to stream channel erosion.

December 2015

The fourth quarter was marked by sustained base flow in Onion Creek. Significant stream flow events were recorded November 26 and December 12-13. The December event was one of the largest in the previous 18 months and resulted in out-of-bank flows for a short period. Water samples were collected on a routine basis during baseflow conditions and at multiple time throughout the duration of storm flows. Samples have been analyzed for sediment, nitrate, total nitrogen, phosphate, and total phosphorus concentrations, with results used for estimating nutrient and sediment export. Stream bank erosion pins were measured at six intensively monitored sites in October, November, and December.  A significant accomplishment this quarter was the completion of a “control stretch” within Onion Creek for more accurate stream discharge monitoring. To provide a stable cross section, stream banks were reshaped and rock placed within the stream and up the banks. This construction required permit approval from local, state, and federal entities.

September 2015

The third quarter of 2015 was marked by low but sustained base flow in Onion Creek. This is in contrast to the same period in 2013 and 2014 when the stream had zero flow. Significant stream flow events were recorded on June 26, August 9, and August 28. Water samples were collected on a routine basis during baseflow conditions, and multiple times throughout the duration of storm flows. Samples have been analyzed for sediment, nitrate, total nitrogen, phosphate, and total phosphorus concentrations, with results used for estimating nutrient and sediment export. Stream bank erosion pins were measured at six intensively monitored sites in July, August, and September.

June 2015

The first quarter of 2015 was marked by sustained base flow in Onion Creek, in contrast to the same period the past two years when the stream had zero flow. The ice thawed in late March, but there were no significant overland flow or high stream flow events. As a result, suspended sediment concentrations were low throughout this period. Water samples for nutrient and sediment were sampled regularly throughout the quarter, and data analysis is ongoing. The Onion Creek stream bank recessions continues to be monitored via erosion pin measurements. 

March 2015

A project in the Onion Creek Watershed is designed to evaluate the effectiveness of conservation practices in reducing sediment and nutrient flux. The final quarter of 2014 was marked by unusually cold temperatures, causing the stream to freeze earlier than normal. Water samples for nutrient and sediment were collected about 15 times throughout the quarter when ice was not present. Samples were analyzed for sediment phosphate, total phosphorus, nitrate and total nitrogen concentrations. The Onion Creek stream bank recession was monitored via erosion pin measurements. Average stream bank recession was estimated at .71 cm since the previous measurement in June.

December 2014

The overall goal of a project in the Onion Creek Watershed is to add to understanding of the importance of stream bed and bank erosion in watershed sediment and nutrient flux. Average daily stream flows within Onion Creek from July to September 2014 were the highest observed over three years of monitoring. Water samples were collected routinely during base flow conditions and at multiple times throughout the duration of storm flows. Samples are being analyzed for sediment and nutrients. Monthly erosion pin measurements were taken at six intensively monitored sites. Analysis of stream bank soils for phosphorus content and bulk density is ongoing.

September 2014

The objectives of a project in the Onion Creek Watershed are to measure watershed sediment flux, estimate sediment (and P) from stream bank erosion, gully erosion, sheet and rill erosion and any other erosion sources in the watershed. Average daily and peak stream flows within Onion Creek during April-June 2014 were the highest observed over three years of monitoring. The watershed received more than 16 inches of rain during May and June, with the highest rainfall total more than three inches on May 20. With already high soil moisture, this event generated out-of-bank discharge May 20-21. Samples were collected and are being analyzed for sediment and nutrients. Initial results show suspended sediment concentrations were the highest recorded over three years of monitoring. Monthly erosion pin measurements are taken at six intensively monitored sites. In April, erosion pins were measured at all 27 sites. Preliminary estimates of total stream bank recession rates showed bank erosion during the May 20-21 event accounted for nearly half of the total stream bank erosion over the three years of monitoring.

June 2014

The objectives of a project in the Onion Creek Watershed are to measure watershed sediment flux, estimate sediment (and P) from stream bank erosion, gully erosion, sheet and rill erosion and any other erosion sources in the watershed. The project will evaluate the effectiveness of conservation practices established. After a dry period when stream flow stopped, Onion Creek began to flow again this spring, and sampling of stream flow resumed. Samples were collected during a high flow event due to snow melt in early March, and are being analyzed for sediment and nutrients. Nitrate levels in early March were around 2 parts per million or less at the watershed outlet. After a rain event April 2, nitrate levels rose to over 20 parts per million. Monthly erosion pin measurements are taken at six intensively monitored sites. In April, erosion pins were measured at all 27 sites. Routine water quality collection sites have been moved to align with bridge sensors established by the Iowa Flood Center, and arrangements made for use of stream stage data from these sensors.

March 2014

In October, erosion pins previously installed in 27 locations within the Onion Creek Watershed were measured to determine stream bank recession rates. Analysis of soil samples for soil texture and total phosphorus content is ongoing. A permanent water-quality sampling site is in place. When spring flows begin, water testing will be used to help better understand how much of the sediment and P delivered to surface waters originates from stream bed and bank erosion.

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