The South Fork McKenzie River (SFMR) in western Oregon, USA hosts one of the largest Stage 0 stream restoration projects implemented to date. Stage 0 refers to a multichannel planform with strong hydrologic connectivity to the adjacent floodplain and surface–subsurface connectivity. Stage 0 restoration was implemented on a 900‐m‐long reach of the SFMR by re‐grading the channel and floodplain using 65,000 m3 of sediment to raise the channel bed. Thousands of large logs were added and the ends of some logs were buried in the sediment to provide foundations for future log jams. Our primary objective is to present a monitoring protocol based on randomly located sampling plots. We also analyze results from 2 years of data collection since project implementation. Within each plot, we measured canopy cover, wood volume, flow depth and velocity, organic cover (area covered by coarse and fine organic material), and substrate grain size. We used intracluster correlation coefficients and variance of measured variables to assess heterogeneity at three spatial scales: within plots, between adjacent plots, and across the entire site. Here, we evaluate changes in the first 2 years after restoration (i.e., not pre‐ vs. post‐restoration). We hypothesized that heterogeneity within a plot would decrease as the plot adjusted to local‐scale hydraulics and sediment and particulate material transport. We hypothesized that heterogeneity would increase between adjacent plots and across the entire site. We found that spatial heterogeneity of geomorphic variables decreased within plots. Heterogeneity of organic cover, sediment size, and flow depth increased between adjacent plots, although other variables did not change. Site‐scale heterogeneity decreased for all variables except organic cover and substrate. We interpret the observed geomorphic responses to reflect decreased longitudinal connectivity and increased lateral and vertical connectivity at the restoration site.
Restoration aimed at rewetting the valley floor has the potential to increase organic carbon stock in the form of floodplain soil carbon, downed wood, and riparian vegetation. The primary goal of stream restoration is typically to restore habitat or maintain balance between natural ecosystem function and human land use. Although many benefits result from stream restoration, the carbon sequestration potential of different restoration approaches in diverse geographic settings has not yet been quantified. We investigate the carbon storage potential of restored stream segments (known as treatment segments) relative to otherwise analogous degraded and reference segments. We develop a conceptual framework to identify the conditions that maximize carbon storage in relation to characteristics of the river corridor and specific restoration practices and propose response surfaces for carbon storage. We illustrate application and quantification of the conceptual framework using data from a pilot study of treatment, degraded, and reference stream segments along two streams in Oregon, United States. The conceptual model is designed to help managers identify levels of hydrologic connectivity, channel and floodplain dynamics, floodplain vegetation, and other variables that may optimize carbon storage at a treatment site.
We use field measurements and airborne LiDAR data to quantify the potential effects of valley geometry and large wood on channel erosional and depositional response to a large flood (estimated 150‐year recurrence interval) in 2011 along a mountain stream. Topographic data along 3 km of Biscuit Brook in the Catskill Mountains, New York, USA reveal repeated downstream alternations between steep, narrow bedrock reaches and alluvial reaches that retain large wood, with wood loads as high as 1261 m3 ha−1. We hypothesized that, within alluvial reaches, geomorphic response to the flood, in the form of changes in bed elevation, net volume of sediment eroded or aggraded, and grain size, correlates with wood load. We hypothesized that greater wood load corresponds to lower modelled average velocity and less channel‐bed erosion during the flood, and finer median bed grain size and a lower gradation coefficient of bed sediment. The results partly support this hypothesis. Wood results in lower reach‐average modelled velocity for the 2011 flood, but the magnitude of change in channel‐bed elevation after the 2011 flood among alluvial and bedrock reaches does not correlate with wood load. Wood load does correlate with changes in sediment volume and bed substrate, with finer grain size and smaller sediment gradation in reaches with more wood. The proportion of wood in jams is a stronger predictor of bed grain‐size characteristics than is total wood load. We also see evidence of a threshold: greater wood load correlates with channel aggradation at wood loads exceeding approximately 200 m3 ha−1. In this mountain stream, abundant large wood in channel reaches with alluvial substrate creates lower velocity that results in finer bed material and, when wood load exceeds a threshold, reach scale increases in aggradation. This suggests that reintroducing small amounts of wood or one logjam for river restoration will have limited geomorphic effects. © 2020 John Wiley & Sons, Ltd.
Extreme storms in forested environments commonly increase inputs of coarse particulate organic matter (CPOM) and large wood (LW) to streams. Protruding boulders and bedforms, mid‐channel bars, and standing trees can trap CPOM and LW. These organic accumulations can become large enough to span the bankfull channel width, or the accumulations can be predominantly along the channel margins. We refer to both types of accumulations as transient organic jams (TOJs). TOJs can create diverse geomorphic and ecological effects in channel and floodplain environments. We use data collected from mountain streams of the Luquillo Mountains of north‐eastern Puerto Rico following September 2017 Hurricanes Irma and Maria. We examine the location, characteristics, and geomorphic effects of TOJs in channel segments representing diverse drainage areas and channel gradients. We ask three questions: (a) Does the downstream spacing of TOJs correlate with variables such as drainage area or channel gradient? (b) What variables best predict the volume of organic matter within individual TOJs or within a channel segment? And (c) is there a threshold within a river network that separates channel segments with channel‐spanning versus marginal TOJs? Datasets include multiple TOJs along each of 12 stream segments and presence/absence of channel‐spanning TOJs along an additional six streams. Data analysis with multiple linear regressions indicates that downstream spacing, average volume, and total volume per channel length of TOJs correlate significantly with bankfull channel width. Using the akaike information criterion with correction (AICc) model selection method, Strahler stream order has the most influence on the probability of TOJs being marginal or spanning.
Floodplain restoration can enhance the capacity for carbon sequestration by facilitating higher water tables, deposition of fine sediment, and increased input and residence time of organic matter. We measured floodplain soil organic carbon stocks in nine stream restoration projects across the western United States and compared them to nearby degraded and reference condition floodplains. Degraded floodplains had the lowest soil mean carbon stocks in the majority of floodplains measured (range 161–894 Mg C/ha), and reference stocks had the highest stocks (range 391–904 Mg C/ha) of those with statistically significant differences between the three categories. Across all sites measured, stream restoration sites, referred to as treatment sites, had stocks (range 203–1028 Mg C/ha) similar to degraded condition floodplains but the largest range. When modeled under degraded conditions, four out of nine of the treatment sites had significantly higher OC stocks than predicted. Climate and geologic variables are most influential in predicting carbon stocks, and floodplains in the interior western USA have the highest carbon stocks. As the demand for carbon sequestration increases due to climate change, ecologically responsible floodplain restoration provides a significant opportunity for carbon storage. However, despite the statistically significant relationships we observed in this dataset, the variations within the data in relation to degraded/treatment/reference categories illustrate the uncertainties in quantifying the effects of restoration on floodplain carbon stocks.
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