Streambank Soil and Phosphorus Losses Under Different Riparian Land‐Uses in Iowa¹

Abstract

Phosphorus and sediment are major nonpoint source pollutants that degrade water quality. Streambank erosion can contribute a significant percentage of the phosphorus and sediment load in streams. Riparian land-uses can heavily influence streambank erosion. The objective of this study was to compare streambank erosion along reaches of row-cropped fields, continuous, rotational and intensive rotational grazed pastures, pastures where cattle were fenced out of the stream, grass filters and riparian forest buffers, in three physiographic regions of Iowa. Streambank erosion was measured by surveying the extent of severely eroding banks within each riparian land-use reach and randomly establishing pin plots on subsets of those eroding banks. Based on these measurements, streambank erosion rate, erosion activity, maximum pin plot erosion rate, percentage of streambank length with severely eroding banks, and soil and phosphorus losses per unit length of stream reach were compared among the riparian land-uses. Riparian forest buffers had the lowest streambank erosion rate (15-46 mm ⁄ year) and contributed the least soil (5-18 tonne ⁄ km ⁄ year) and phosphorus (2-6 kg ⁄ km ⁄ year) to stream channels. Riparian forest buffers were followed by grass filters (erosion rates 41-106 mm ⁄ year, soil losses 22-47 tonne ⁄ km ⁄ year, phosphorus losses 9-14 kg ⁄ km ⁄ year) and pastures where cattle were fenced out of the stream (erosion rates 22-58 mm ⁄ year, soil losses 6-61 tonne ⁄ km ⁄ year, phosphorus losses 3-34 kg ⁄ km ⁄ year). The streambank erosion rates for the continuous, rotational, and intensive rotational pastures were 101-171, 104-122, and 94-170 mm ⁄ year, respectively. The soil losses for the continuous, rotational, and intensive rotational pastures were 197-264, 94-266, and 124-153 tonne ⁄ km ⁄ year, respectively, while the phosphorus losses were 71-123, 37-122, and 66 kg ⁄ km ⁄ year, respectively. The only significant differences for these pasture practices were found among the percentage of severely eroding bank lengths with intensive rotational grazed pastures having the least compared to the continuous and rotational grazed pastures. Row-cropped fields had the highest streambank erosion rates (239 mm ⁄ year) and soil losses (304 tonne ⁄ km ⁄ year) and very high phosphorus losses (108 kg ⁄ km ⁄ year).(KEY TERMS: riparian areas; streambank erosion; soil and phosphorus losses; best management practices; grazing practices; nonpoint source pollution.) Zaimes, George N., Richard C. Schultz, and Thomas M. Isenhart, 2008. Streambank Soil and Phosphorus Losses Under Different Riparian Land-Uses in Iowa. Journal of the American Water Resources Association (JAWRA) 44(4):935-947.

Phosphorus source—sink relationships of stream sediments in the Rathbun Lake watershed in southern Iowa, USA

“…Mean TP and Mehlich-3-extractable P, however, were similar in bed and bank sediments. Total P concentrations in the bank sediments were similar to those found in stream bank soils throughout the Rathbun Lake watershed as reported by Tufekcioglu et al (2012), but somewhat lower than the 360-555 mg P kg −1 range that Zaimes et al (2008) found in other watersheds in southeast Iowa. The range of TP concentrations in stream bed sediments collected by Zaimes et al (2008) was also higher (389-964 mg P kg…”

“…) than we found in samples from the four sites (Table 3), but Zaimes et al (2008) reported that samples had significant amounts of fine-textured material, which tend to have higher TP concentrations.…”

“…When the study of Bear et al (2012) was initiated, surface horizons (0-20 cm) of soils at each site were sampled to determine inherent soil properties. Of interest, total P (Crosland et al 1995) in the soils ranged from 267 to 349 mg kg , which is comparable to total P (TP) measured in other Iowa watersheds with similar soil types (Zaimes et al 2008;Nellesen et al 2011). Additional information describing the 13 study sites is available from Bear et al (2012) and Tufekcioglu et al (2012).…”

“…Nitrate-N concentrations greater than 6.0 mg L −1 are common in surface waters in Iowa, although many watersheds have significantly more row crop production than present in the Rathbun Lake watershed (Jaynes et al 1999;Tomer et al 2008;Hatfield et al 2009 , the streams in the watershed carry significant amounts of sediment, likely a result of stream bank erosion (Zaimes et al 2008;Tufekcioglu et al 2012). Seasonal sediment and nutrient loads were not calculated as sufficient stream flow data are not available.…”

“…In Iowa, Downing and Kopaska (2000) showed that watersheds with greater portions of land in grazed pastures had higher sediment and P in surface water than did watersheds in cropland, although input pathways were not identified. More recent research in Iowa has indicated that pasturelands can be major contributors of sediment and nutrients to surface water resources if improper grazing management reduces vegetative cover (Zaimes et al 2004;Zaimes et al 2008;Nellesen et al 2011).…”