Sediment and phosphorus loads from streambank erosion and failure: A source of legacy phosphorus in watersheds

Streambank erosion may be one pathway for sediment and nutrient loading to streams but insufficient data exists on the magnitude. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. Objectives of this research include (i) reviewing current knowledge on streambanks as P loading sources and identifying future research needs, (ii) quantifying the amount of streambank erosion throughout a sensitive watershed in eastern Oklahoma, (iii) estimating the benefit of vegetation on reducing streambank erosion, (iv) determining the importance of mass wasting in this system, (v) analyzing the appropriateness of limited monitoring points to determine watershed sediment load, (vi) quantifying the magnitude of and spatial distribution of streambank phosphorus concentrations along a stream system in a watershed with historical poultry litter application, (vii) quantifying the amount of water soluble phosphorus (WSP) and total phosphorus (TP) entering the stream from streambanks, and (viii) comparing streambank P concentrations and loading between two unique streams in the same ecoregion. For Spavinaw Creek, it was estimated that the total soil mass eroded from 2003 to 2013 was 727 x106 kg, average bank retreat was 2.5 m yr-1, and 1.5 x 103 kg WSP and 1.4 x 105 kg TP loaded. Statistical analysis showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks. Bank retreat was somewhat positively correlated with stream discharge, suggesting that mass wasting plays a role in streambank erosion within this watershed. Selection of random sites and scaling up to watershed scale greatly underestimated the actual erosion and loading rates. Comparison of P loading between the two systems showed that WSP in one was an order of magnitude higher while TP was on the same order of magnitude. Streambank P loading rates are dependent on the stream system; therefore each stream needs to be individually studied in order to gain a better understanding of the specific loadings from streambanks. Future research is needed on dynamics between different P pools and the integrated streambank erosion processes.