Assessing Potential Controls on River Bead Functionality in Mountain Streams

Published: 10 September 2025| Version 1 | DOI: 10.17632/xx66nsvk56.1
Contributors:
Katherine Larkin,

Description

We evaluated flux attenuation potential, referred to as functionality, in laterally extensive, storage-dominated river reaches known as ‘beads’. Bead functionality was evaluated as a relationship between driver variables, which directly measure or measure proxies of geomorphic and biotic system inputs, and response variables, which are proxy variables believed to influence flux travel time and storage magnitude, assuming that functional beads contribute to higher travel times and storage magnitudes at the network scale. Geomorphic driver variables include drainage area, catchment slope, elevation, land cover and precipitation metrics, which represent water inputs into the stream corridor, as well as delta normalized burn index (dNBR) and catchment slope, which further represent sediment inputs into the stream corridor. Biotic driver variables within each bead include wood load, beaver modifications, and type of riparian vegetation. Response variables include normalized difference vegetation index (NDVI), normalized difference water index (NDWI), patch density, and total sinuosity. Driver and response variables were measured through a mixture of fieldwork and remote data for 52 beads in 27 catchments in the Colorado Front Range, USA. Statistical analyses examined relationships between drivers and responses and the effectiveness of grouping the beads in different ways (by dominant vegetation and by elevation). Analyses suggest that bead functionality is most strongly linked to bead ratio, or the ratio of bead size to catchment size. Functional beads are larger relative to catchment size. In addition, beads can be efficiently grouped by dominant vegetation; these different types of beads reflect significant differences in catchment geometry, geomorphic inputs, and biotic inputs, and display significant differences in bead geometry. Although functionality is the complex result of numerous factors and may require case-by-case assessment efforts, restoration of channel-floodplain connectivity and facilitating greater retention of water will enhance river restoration by increasing the width of the active floodplain. Investigating drivers of functionality provides a crucial link between system inputs, restoration action, and desired response, allowing plans to be tailored to address targets. Because bead position and geometry cannot be feasibly modified, the functionality framework can be used to identify sites with the greatest potential for restoration.

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Fieldwork was completed with the objective of verifying bead geometry assessed remotely, documenting vegetation, and assessing wood volume and beaver modifications where present. Active channel width and flow depth were measured with a Laser Technology TruPulse 360 laser rangefinder (+ 0.1 m resolution) and a hand tape, respectively, and bed substrate was assessed visually. Bead starting points (“inlets”) and ending points (“outlets”) were collected as points with a Garmin eTrex10 (+ 3 m horizontal accuracy). Large wood (LW), defined as length > 1 m and diameter > 10 cm, and logjams, defined as a cluster of three or more pieces of large wood, were documented in the field. Logjams and wood pieces were assigned a GPS location, width and length, diameter, and azimuthal direction, and characterized for their decay level, configuration, and complexity level. Floodplain wood pieces, as well as wood of smaller lengths and/or widths, were not included in this study. All wood surveyed was at least partly within the bankfull channel. GPS locations and valley-bottom extent of both active and relict beaver berms were mapped in the field for each bead. Minimum berm count and primary channel length were used to calculate berm density, or the number of berms per kilometer of valley length. USGS 3DEP 1-meter digital elevation models (DEMs) were sourced from OpenTopography for elevation and topographic data. ArcGIS Pro and Google Earth Pro were used to estimate bead-scale geometric variables (area, slope, floodplain-channel width ratio). The USGS StreamStats Batch Processing Tool was used to calculate geomorphic variables influencing water inputs, as well as surface water storage, approximate precipitation, 2-year, and 100-year flood discharges. Thirty-meter Landsat 8 Collection-2 Level-2 satellite imagery from early July 2020 and early September 2021 were used as pre- and post-fire data. Pre-fire NBR and post-fire NBR rasters were created in ENVI, and ArcGIS Pro produced the finalized output differencing the two rasters. Zonal histograms quantified severity distributions across the entire burn scar, summarized into three metrics for each catchment: the proportion of the catchment burned at high severity (dNBR >0.66), the proportion of the catchment burned, and the proportion of high severity burn to total burn. PlanetScope 3-m RGB-NIR imagery from July 2021 was used to calculate NDVI and NDWI. NDVI and NDWI rasters and summary statistics were calculated in ArcGIS Pro. Patches were identified through field observation,, delineated in Google Earth Pro, and analyzed in ArcGIS Pro. The target metric, patch density, was calculated as the number of patches per square kilometer of bead area. The number of continuous patches was also counted for each bead. Valley and primary/secondary channel lengths were calculated in Google Earth Pro.

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Remote Sensing, Fluvial Geomorphology, Mountain Stream

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