H52A-01 10:20h
The Restoration Response Function as an Organizing Tool for Evaluating Success of River Management Programs
The national effort to reverse undesired environmental conditions on regulated rivers involves substantial societal costs that inevitably force prioritization of reaches for rehabilitation and of restoration techniques. Societal costs are associated with transformation of the flow and sediment transport regime, physical manipulation of the channel and floodplain, administrative costs of river management agencies, research costs, costs of new technology such as temperature control or sediment by-pass, and costs of reduced hydropower production. One strategy for assessing costs and assigning priorities is to assemble data about restoration program success and failure within some organizing framework. The restoration response function is one such organizing framework. This function defines the relationship between the costs of flow and sediment alteration and the results of those actions in reversing undesired river conditions. Development of these functional relations should be an essential element in a national assessment of stream restoration programs. Numerous river management programs in the Colorado River basin provide an opportunity to compare restoration response functions and identify those river segments where greater environmental gains are afforded by smaller societal costs. Restoration response functions differ significantly in conditions of sediment deficit, such as downstream from Glen Canyon Dam, and in conditions of sediment surplus, such as exist throughout much of the Green River in Utah. In conditions of severe sediment deficit, large costs are sustained without significant results. Greater benefits in reversing undesired environmental conditions are possible for smaller costs in conditions of sediment surplus. Thus, the effort to rehabilitate and restore aggrading segments of the Colorado River in the Upper Basin are relatively efficient economically while the costs of similar efforts in the Grand Canyon are relatively inefficient.
H52A-02 10:35h
Rehabilitation Experiments in the Context of Adaptive Management, Lower Missouri River
Morphology of the highly engineered Lower Missouri River (Sioux City, Iowa to St. Louis, Missouri) is nearly independent of flow regime because channelization has greatly reduced opportunity for channel adjustment. Rehabilitation efforts have therefore focused on altering channel form directly rather than altering flow regime. Rehabilitation sites offer the potential for experimental studies to address geomorphic adjustments and tradeoffs in form and flow in river recovery. Implementation of experiments remains a challenge, however, because increased scientific understanding is not universally valued by all stakeholders participating in the adaptive management process. Nevertheless, some rehabilitation sites have been utilized for opportunistic geomorphic experiments. Engineered rehabilitation projects typically seek to increase diversity of habitats, with emphasis on increasing areas of shallow, slow current velocity that are much diminished from their historical extent and which are thought to be especially important to recovering ecological processes. Projects fall into two broad categories: those that increase width of the channel or diversity of habitats between the high banks, and those that create or re-create side-channel chutes. Early results from two contrasting experiments in side-channel chute development indicate that a) the experimentally uncontrolled sequence and magnitude of hydroclimatic events have a substantial effect on morphogenesis, and b) geomorphic and ecological responses are highly influenced by reach-scale spatial patterns of sediment and large-woody debris transport that determine fluxes into chutes. After less than a year of monitoring of channel-widening experiments, we have only preliminary results, but we have developed a keen appreciation for experimental approaches in an extremely dynamic and spatially diverse river. Particular challenges are developing robust, cost-effective, geomorphic performance metrics and designing experiments where replicates cannot be defined and management timeframes are short relative to timeframes of geomorphic processes.
H52A-03 10:50h
Sediment Studies Refute EIS Hypothesis, While Most Fundamental Process Questions Remain Unanswered: An Update on Experiments in Grand Canyon
For three decades, sediment researchers have pondered the question of whether or not operations at Glen Canyon Dam could be adjusted to maintain downstream sand resources in Grand Canyon. Prior to the era when managed floods were proposed as a strategy to conserve sand inputs below the dam, Laursen et al. (1976) concluded that erosion of sandbars below the dam would be an inevitable, yet protracted post-dam process. Despite this earliest conclusion, the operational strategy for sandbar maintenance since 1996, has been based on two hypotheses: first, much of the sand introduced by tributaries downstream from the dam can accumulate in the channel over multiple years under operations associated with average-to-below average hydrology; and second, controlled floods can move that accumulated sand from the channel bed to shorelines, thereby rebuilding bars in a sustainable manner. Recent work has shown that the first hypothesis is false (Rubin et al., 2002). High resolution data for the ecosystem sand mass balance between 1999 and 2004, indicate no accumulation of tributary sand inputs in the main channel, despite a drought resulting in minimum annual release volumes from 2000 through 2004. Sandbar data also indicate that erosion has not been mitigated by re-operation strategies since 1991. On the basis of these data, researchers have again identified uncertainty regarding a flow strategy that will result in sustainable sandbars. If a successful flow strategy can not be devised, then managers may have to choose between abandoning sandbar restoration objectives, or pursuing sediment augmentation. Experimental fluctuating-flow treatments are also being evaluated for their potential to limit populations of introduced rainbow trout, yet these options are already known to increase sand export. While many institutional barriers to large-scale sediment experiments in Grand Canyon have recently been bridged through a science-based, adaptive management approach, protracted drought throughout the Upper Colorado River Basin now poses a natural barrier to testing the key sediment hypothesis. Downstream sand production from the Paria River remains at its lowest level in 80 years, while water storage in Lake Powell approaches 40 percent of capacity. As scientists wait out delays in sediment experimentation forced by the current drought, managers have already approved limited sediment augmentation feasibility studies aimed at identifying options for managing physical habitats. With regard to conventional thinking about regulation and management of natural hydrologic systems, some important lessons may be learned from the current situation. Rubin, D.M., Topping, D.J., Schmidt, J.C., Hazel, J., Kaplinski, M. and Melis, T.S., 2002, Recent Sediment Studies Refute Glen Canyon Dam EIS Hypothesis: {\it Eos}, vol. 83, no. 25, p. 273-278. Laursen, E.M., Ince, S. and Pollack, J., 1976, On Sediment Transport Through the Grand Canyon, Proceedings of the 3rd Federal Interagency Sedimentation Conference, Denver, CO, vol. 1, p. 4-76 - 4-87.
H52A-04 INVITED 11:05h
Using high-resolution suspended-sediment measurements to infer changes in the topographic distribution and grain size of bed sediment in the Colorado River downstream from Glen Canyon Dam
Eddy sandbars and other sandy deposits in and along the Colorado River in Grand Canyon National Park (GCNP) were an integral part of the pre-dam riverscape, and are still important for habitat, protection of archeological sites, and recreation. Recent work has shown that eddy bars are dynamic landforms and represent the bulk of the ecosystem's sand reserves. These deposits began eroding following the 1963 closure of Glen Canyon Dam that reduced the supply of sand at the upstream boundary of GCNP by about 94% and are still eroding today. Sand transport in the post-dam river is limited by episodic resupply from tributaries, and is equally regulated by the discharge of water and short-term changes in the grain size of sand available for transport (Rubin and Topping, {\it WRR}, 2001). During tributary floods, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. This prohibits the computation of sand-transport rates in the Colorado River using stable relations between water discharge and sand transport (i.e., sediment rating curves) and requires a more continuous method for measuring sand transport. To monitor suspended sediment at higher (i.e., 15-minute) resolutions, we began testing a laser-acoustic system at four locations along the Colorado River in Grand Canyon in August 2002. Because they are much easier to acquire, the high-resolution suspended-sediment datasets collected using the laser-acoustic systems greatly outnumber (by $>$5 orders of magnitude) direct grain-size measurements of the upstream bed sediment. Furthermore, suspension processes effectively provide an average "sample" of the bed sediment on the perimeter of the upstream channel and the underwater portions of the banks and eddy bars. Thus, it is advantageous to analyze suspended-sediment concentration and grain-size data to infer changes in the topographic distribution and grain size of the upstream bed sediment. Rubin and Topping (2001) developed and tested a theory-based technique that can be used for this purpose. Their parameter "$\beta$" is a nondimensional measure of the average bed-surface grain-size that interacts with the suspended sediment in the flow. Analyses of the laser-acoustic datasets indicate that, when the Colorado River is relatively enriched with respect to finer sand, the discharge of water, and the concentration and grain size of the suspended sand are all positively correlated. During these periods, $\beta$ is negatively correlated with discharge, indicating that the sand on the bed is finer at higher elevations along the banks. Although water discharge and the concentration of suspended sand remain positively correlated when the river is relatively depleted in finer sand, grain size of suspended sand then becomes negatively correlated with both the discharge of water and suspended-sand concentration. The greater decrease in $\beta$ as a function of discharge demonstrates the decrease in the grain size of the bed sand as a function of elevation is much greater under sand-depleted conditions than under sand-enriched conditions. Thus, these analyses indicate that, during periods of erosion, sand is winnowed preferentially from lower elevations along the river.
H52A-05 11:25h
Climate-Related Flood and Sediment Transport From the Paria River to Grand Canyon: The Role of Multiple Time Scales
Since the 1963 closure of Glen Canyon Dam, the sole major supplier of sand to the Colorado River in the upper portion of Grand Canyon is the Paria River, which supplies about 6% of the pre-dam supply of sand at the upstream boundary of Grand Canyon National Park. Sand is delivered by the Paria River during short-duration ($<$ 24 hours), large magnitude (up to 300 m$^{3}$s$^{-1}$) floods that occur primarily during the warm season (July-October). The planning and decision processes in the Glen Canyon Dam Adaptive Management Program (AMP) strive to balance numerous, often competing, objectives, such as, water supply, hydropower generation, low flow maintenance, maximizing conservation of the tributary supplied sediment, endangered species recovery, and cultural resources. In this work, we focus on a key concern identified by the AMP, related to the timing and volume of sediment input into Grand Canyon. Adequate sediment inputs into the Canyon combined with active management of the timed releases from Glen Canyon Dam support the restoration and maintenance of sandbars and instream ecology. For the Paria River, we relate the climatic drivers of episodic to interdecadal variations to the observed changes in the flood magnitude, timing and spatial scales as they affect the sediment inputs to the Colorado River. Variability in regional precipitation distribution on multiple time scales is diagnosed with emphasis on understanding the relative role of East Pacific tropical storms, North Pacific sea surface temperatures, and subtropical moisture sources. Better understanding of the coupled climate-hydrologic variations on multiple time scales is increasingly recognized as critical input for adaptive management (both passive and active). In collaboration with the AMP, this work deliberately identifies the entry-points for predictive hydroclimatic information at appropriate lead times. From the standpoint of this active adaptive management program, lead climate information allows scientists and managers to anticipate geomorphic response from critical tributaries, that in turn trigger large-scale, experimental releases from Glen Canyon Dam.
H52A-06 11:40h
Erosion characteristics of fine-grained, beach-building sediment along the Colorado River in Grand Canyon
In the Grand Canyon segment of the Colorado River, eddy sandbars, which form in lateral recirculation eddies, are important for endangered fish habitat, riparian habitat, protection of archeological sites, and recreation. By virtue of the 1963 closure of Glen Canyon Dam, sediment (i.e., sand, silt, and clay) supply to the Colorado River at the upstream boundary of Grand Canyon National Park has been reduced to about 5% of the pre-dam supply. This has caused substantial reduction in the size of eddy sandbars. The major supplier of sediment in the first 123 km downstream from Glen Canyon Dam is the Paria River, and its sediment consists mainly of clay, silt, and finer sand. During large floods on the Paria River, about 50% of the load is silt and clay, and the median size of the sand is about 0.11-0.12 mm. In order to restore the eroded eddy sandbars in the upper portion of Grand Canyon, an experimental controlled flood, i.e., Beach Habitat Building Flow (BHBF), has been proposed following enrichment of the sediment supply by flooding on the Paria River. Deposits produced by this BHBF should be fine-grained and cohesive. Understanding the sediment-transport behavior of this cohesive sediment is essential for the prediction and evaluation of the influence of the BHBF on rebuilding bars and increasing turbidity in the main channel. In this study, cohesive sediment samples of beach bars were collected from bars in the Colorado River in the Lake Mead delta. Laboratory experiments have tested the bulk density, erosion rate, and critical shear stress of these collected samples. The erosion rate of each sample was tested several times at different boundary shear stresses in a laboratory flume, allowing for estimation of the critical shear stress. Samples were placed in a 10-cm diameter cylinder below the flume. The sample was pushed out of the cylinder as it was eroded, such that the sample surface remained at the same height as the flume floor. Boundary shear stresses were estimated from near-bed, acoustic Doppler velocimetry measurements. Erosion rates were measured by comparing digital pictures of a laser line on the sediment surface at fixed time intervals. Results of experiments show that erosion rates of silt and sand rich samples rapidly increase when bottom shearing stress exceeds about 0.5(N/m$^{2}$), and those rates range from 0.0001(cm/sec) to 0.0008(cm/sec). On the other hand, erosion rates of clay rich samples and organic rich silt are much smaller at these stresses, and very small amounts of erosion is observable at even very low boundary shear stresses. Also, results show that the erosion rate decreases over time for a range of shear stresses.