Paul J. Kinzel

Colorado River sediment transport 2. Systematic bed-elevation and grain-size effects of sand supply limitation

The Colorado River in Marble and Grand Canyons displays evidence of annual supply limitation with respect to sand both prior to (Topping et al., this issue) and after the closure of Glen Canyon Dam in 1963. Systematic changes in bed elevation and systematic coupled changes in suspended-sand concentration and grain size result from this supply limitation. During floods, sand supply limitation either causes or modifies a lag between the time of maximum discharge and the time of either maximum or minimum (depending on reach geometry) bed elevation. If, at a cross section where the bed aggrades with increasing flow, the maximum bed elevation is observed to lead the peak or the receding limb of a flood, then this observed response of the bed is due to sand supply limitation. Sand supply limitation also leads to the systematic evolution of sand grain size (both on the bed and in suspension) in the Colorado River. Sand input during a tributary flood travels down the Colorado River as an elongating sediment wave, with the finest sizes (because of their lower settling velocities) traveling the fastest. As the fine front of a sediment wave arrives at a given location, the bed fines and suspended-sand concentrations increase in response to the enhanced upstream supply of finer sand. Then, as the front of the sediment wave passes that location, the bed is winnowed and suspended-sand concentrations decrease in response to the depletion of the upstream supply of finer sand. The grain-size effects of depletion of the upstream sand supply are most obvious during periods of higher dam releases (e.g., the 1996 flood experiment and the 1997 test flow). Because of substantial changes in the grain-size distribution of the bed, stable relationships between the discharge of water and sand-transport rates (i.e., stable sand rating curves) are precluded. Sand budgets in a supply-limited river like the Colorado River can only be constructed through inclusion of the physical processes that couple changes in bed-sediment grain size to changes in sand-transport rates. In some rivers the upstream supply of sediment is in equi- librium with the upstream supply of water, whereas in others, the upstream supply of sediment is decoupled, either com- pletely or partially, from the upstream supply of water. In the first type of river, changes in sediment transport are controlled by changes in the discharge of water, whereas in the second (and perhaps more common) type of river, changes in sedi- ment transport are also coupled to changes in sediment grain size. In this paper we investigate the systematic changes in bed elevation, sediment transport, and sediment grain size that occur in response to changes in the upstream supply of sand in a river with an intermittent limited supply of sand, specifically the Colorado River in Marble and Grand Canyons (Figure 1). To develop an intuitive understanding of the linkage be- tween sediment grain size and the upstream supply of sediment in a river, it is informative to first examine sediment-transport

Changes in Agriculture and Abundance of Snow Geese Affect Carrying Capacity of Sandhill Cranes in Nebraska

Abstract The central Platte River valley (CPRV) in Nebraska, USA, is a key spring-staging area for approximately 80% of the midcontinent population of sandhill cranes (Grus canadensis; hereafter cranes). Evidence that staging cranes acquired less lipid reserves during the 1990s compared to the late 1970s and increases in use of the CPRV by snow geese (Chen caerulescens) prompted us to investigate availability of waste corn and quantify spatial and temporal patterns of crane and waterfowl use of the region. We developed a predictive model to assess impacts of changes in availability of corn and snow goose abundance under past, present, and potential future conditions. Over a hypothetical 60-day staging period, predicted energy demand of cranes and waterfowl increased 87% between the late 1970s and 1998–2007, primarily because peak abundances of snow geese increased by 650,000 and cranes by 110,000. Compared to spring 1979, corn available when cranes arrived was 20% less in 1998 and 68% less in 1999; consequently, the area of cornfields required to meet crane needs increased from 14,464 ha in 1979 to 32,751 ha in 1998 and 90,559 ha in 1999. Using a pooled estimate of 88 kg/ha from springs 1998–1999 and 2005–2007, the area of cornfields needed to supply food requirements of cranes and waterfowl increased to 65,587 ha and was greatest in the eastern region of the CPRV, where an estimated 54% of cranes, 47% of Canada geese (Branta canadensis), 45% of greater white-fronted geese (Anser albifrons), and 46% of snow geese occurred during ground surveys. We estimated that a future reduction of 25% in available corn or cornfields would increase daily foraging flight distances of cranes by 27–38%. Crane use and ability of cranes to store lipid reserves in the CPRV could be reduced substantially if flight distance required to locate adequate corn exceeded a physiological maximum distance cranes could fly in search of food. Options to increase carrying capacity for cranes include increasing accessibility of cornfields by restoring degraded river channels to disperse roosting cranes and increasing wetland availability in the Rainwater Basin to attract snow geese using the CPRV.

Spring Census of Mid‐Continent Sandhill Cranes Using Aerial Infrared Videography

Abstract Aerial infrared videography was used to map spatial distributions of nocturnal sandhill crane (Grus canadensis) flocks and determine crane densities within roosts as an alternative to the currently used diurnal photo-corrected ocular transect method to estimate the size of the mid-continental population. The densities determined from samples taken over the course of a night show variability. Densities measured early in the night (2100 to 2300 hrs) were generally lower than those measured in the time period after midnight and up until cranes prepared to depart their roosts before sunrise. This suggests that cranes may be more active early in the night and possibly still settling into their roosts at this time. For this reason, densities and areas measured later at night and into the early morning were used to estimate population size. Our methods estimated that the annual crane populations along the central Platte River in Nebraska were higher than estimates from the ocular transect method; however both methods showed a similar trend with time. Our population size estimates likely were higher because our methodology provided synoptic imagery of crane roosts along the entire study reach when all cranes had returned to the river, and the nocturnal densities were higher than previous estimates using observations from late evening or early morning. In addition to providing a tool for estimating annual population size, infrared videography can be utilized over time to identify spatial changes in the roosting patterns that may occur as a result of riverine management activities.

Sampling Strategies to Improve Passive Optical Remote Sensing of River Bathymetry

Passive optical remote sensing of river bathymetry involves establishing a relation between depth and reflectance that can be applied throughout an image to produce a depth map. Building upon the Optimal Band Ratio Analysis (OBRA) framework, we introduce sampling strategies for constructing calibration data sets that lead to strong relationships between an image-derived quantity and depth across a range of depths. Progressively excluding observations that exceed a series of cutoff depths from the calibration process improved the accuracy of depth estimates and allowed the maximum detectable depth (dmax) to be inferred directly from an image. Depth retrieval in two distinct rivers also was enhanced by a stratified version of OBRA that partitions field measurements into a series of depth bins to avoid biases associated with under-representation of shallow areas in typical field data sets. In the shallower, clearer of the two rivers, including the deepest field observations in the calibration data set did not compromise depth retrieval accuracy, suggesting that dmax was not exceeded and the reach could be mapped without gaps. Conversely, in the deeper and more turbid stream, progressive truncation of input depths yielded a plausible estimate of dmax consistent with theoretical calculations based on field measurements of light attenuation by the water column. This result implied that the entire channel, including pools, could not be mapped remotely. However, truncation improved the accuracy of depth estimates in areas shallower than dmax, which comprise the majority of the channel and are of primary interest for many habitat-oriented applications.

Bathymetry of Clear Creek Reservoir, Chaffee County, Colorado, 2016

Figure 3. Elevation-volume curve of Clear Creek Reservoir, Chaffee County, Colorado, in 2016 and 1997. In 1997, the Pueblo Board of Water Works carried out a bathymetry survey of the reservoir using photogrammetry to develop the stage-volume relations. In 2016, another bathymetry survey by the U.S. Geological Survey, the Pueblo Board of Water Works, and Colorado Mountain College was carried out using a hand-operated boat-mounted, multibeam echo sounder integrated with a Global Positioning System and a terrestrial survey using real-time kinematic Global Navigation Satellite Systems. El ev at io n, in fe et (N AV D 88 )