Large Rivers III: Posters
Presiding: S Leclair, Department of Earth and Environmental Sciences, Tulane University; M Allison, Department of Earth and Environmental Sciences, Tulane University; R Aalto, University of Washington; J Syvitski, University of Colorado; W Dietrich, University of California, Berkeley
H33A-01 1330h
The Columbia River--on the Leading Edge
On the leading edge of the North American plate, the Columbia River is the largest of the world's 40 or so rivers with drainage areas greater than 500,000 square kilometers to drain toward a convergent plate boundary. This unique setting results in a unique continental river basin; marked by episodic and cataclysmic geologic disturbance, but also famously fecund with perhaps 10 to 16 million salmon historically spawning in its waters each year. Now transformed by dams, transportation infrastructure, dikes and diversions, the Columbia River presents an expensive conundrum for management of its many values. Inclusion of river ecology and geomorphology in discussions of river management is generally limited to observations of the last 200 years-a time period of little natural disturbance and low sediment transport. However, consideration of longer timescales provides additional perspective of historical ecologic and geomorphic conditions. Only 230 km from its mouth, the Columbia River bisects the volcanic arc of the Cascade Range, forming the Columbia River Gorge. Cenozoic lava flows have blocked the river, forcing diversions and new canyon cutting. Holocene eruptions of Mount Mazama (Crater Lake), Mount Hood, Mount St. Helens, and Mount Rainier have shed immense quantities of sediment into the lower Columbia River, forming a large percentage of the Holocene sediment transported through the lower river. Quaternary landslides, perhaps triggered by great earthquakes, have descended from the 1000-m-high gorge walls, also blocking and diverting the river, one as recently as 550 years ago. These geologic disturbances, mostly outside the realm of historical observation and operating at timescales of 100s to 1000s of years in the gorge and elsewhere, have clearly affected basin geomorphology, riverine ecology, and past and present cultural utilization of river resources. The historic productivity of the river, however, hints at extraordinary resilience (and perhaps dependence) of the Columbia River system to such disturbances, many of which are similar to engineered disturbances of the last 200 years.
H33A-02 1330h
Colombian Hydrologic Atlas
Colombia has climatic influences from the Caribbean Sea, the Pacific Ocean and the Atlantic Ocean through the tropical forest of the Amazon basin and the savannas of the Orinoco River. To make things mores complex, the western part is crossed by the Andes Mountains. This work present a systematic effort to estimate the long term averages of the main hydrologic variables using all available information, physical principles and statistical estimation techniques. Topography for instance, is a major control for precipitation, temperature, wind velocity and air humidity. The precipitation field was estimated using a hand made expert map as an input trend for a universal kriging interpolation technique. The estimation of actual evaporation was a major challenge because of the variety of methods and because of conceptual problems related to the long term and regional scale estimation needed, whereas most of the methods are short term point estimation equations of potential evaporation. A careful evaluation of climatic evaporation estimation was carried out, including Turc's, Budyko's and advection aridity methods like Morton's. The results were tested using the long term water balance equation against 200 streamflow gauging stations. Morton's method gave the least estimation error for long term river discharges (average of 20%), with not significant superiority over other methods. Overall, we conclude that the magnitude of errors arises fundamentally from deficiencies in the data and the sparsity of the observations. Besides the long term average fields we estimated floods and low flow fields for different return periods, for the entire river networks of Colombia using scaling ideas and the long-term flow field as the scaling variable. The result is a Hydrologic Atlas of Colombia, in a custom-made geographic information system which extracts basins and integrate the field for any particular site.
http://cancerbero.unalmed.edu.co/~hidrosig/index.php
H33A-03 1330h
Hydrological Data and Maps Based on SRTM Elevation Derivatives at Multiple Scales
Watershed analyses, hydrological modeling, and freshwater conservation planning typically require information on stream networks, watershed boundaries, or drainage routing schemes in digital format. Aiming to provide a new generation of these data at a global extent, World Wildlife Fund is currently developing the HydroSHEDS data set (Hydrological data and maps based on Shuttle elevation derivatives at multiple scales). HydroSHEDS is based on elevation data of the Shuttle Radar Topography Mission (SRTM). As its core data layer, HydroSHEDS provides a seamless near-global drainage direction map at a resolution of 3 arc-seconds (90 meters). From this map, additional products are derived at multiple scales, including hierarchical watershed delineations and topological river networks. To generate HydroSHEDS, the original SRTM elevation data have been hydrologically conditioned in a sequence of automated procedures. Both standard methods of data improvement and newly developed algorithms have been applied, including customized gap filling, filtering, stream burning, and upscaling techniques. Manual corrections were added where necessary. Preliminary quality assessments indicate that the accuracy of HydroSHEDS significantly exceeds that of existing global watershed and river maps. For many regions, and particularly large river basins, the new data can support hydrological assessments at previously inaccessible resolutions and extents.
H33A-04 1330h
A Conceptual Physical Model of Jet Current Turbulence in the Mouth of a River Inflowing Into a Nontidal Enclosed Sea
The results of measurements performed in multiple river mouths, as well as theoretical analysis, have led to revealing a number of special features of mouth currents. This made it possible to formulate a conceptual physical model of free jet current turbulence in the mouth of a river inflowing into a nontidal enclosed sea. Its main principles are as follows: The structure of turbulence is characterized by the presence of a hierarchy of energy supply zones in the velocity spectra. The energy spectra have two basic discrete zones of energy supply, which are related: the first to the hydrodynamic instability of the averaged flow, and the second to the increased friction on the river jet boundaries. These discrete zones are separated by the inertial interval. Moving away from the mouth into the sea (to the bar), the ordinates of the spectral density are decreasing and the zones of additional energy supply move into the domain of higher frequencies. In all sections of the jet flow within the river mouth, the generation of turbulent energy exceeds its dissipation. This excess increases along the jet flow. In all sections, dissipation and generation of turbulent energy increase from the surface to the bottom. The dissipation of energy decreases in the section before the bar, while beyond the bar it rises along the flow, due to the increasing hydraulic resistance at the bar. The longitudinal dimensions of turbulent eddies get smaller with the distance from the mouth gauge, while their orientation is changing from the predominantly vertical-longitudinal rotation over a horizontal axis to the predominantly horizontal-transversal rotation over a vertical axis. With the advance of the river jet from the mouth into the sea, the predominance of the longitudinal dimensions of the eddies changes to the predominance of their transversal dimensions. The farther from the mouth, in the spectra of the river jet current there appear frequency intervals that can be described not by the known Kolmogorov's "-5/3" law but by the "-7/3" law. This feature is due to the increasing friction on the side boundaries of the jet. As the river jet moves beyond the mouth bar, another frequency interval appears in the energy spectra; it can be described by the "-3" law. This is attributed to the consumption of energy by the river jet flow for overcoming the ascending forces when the process of vertical mixing of river and sea water masses is going on beyond the bar.
H33A-05 1330h
Modeling Longitudinal Profiles and Downstream Fining in Large, Sand-bed Rivers
A well-known characteristic of many rivers is a decrease in the characteristic bed-sediment grain diameter (e.g. the median) in the downstream direction. This characteristic, often termed downstream fining, is typically accompanied by a downstream decrease in bed slope, i.e. a concave upward longitudinal profile. Two classical examples of these phenomena in large, sand-bed rivers are the lower Mississippi River, USA, and the middle Fly River, Papua New Guinea. In order to study the mechanisms that control longitudinal profile development and downstream fining in large, sand-bed rivers, we developed a numerical model capable of simulating these phenomena over space scales of hundreds of kilometers and time scales of thousands of years. A key aspect of the numerical model formulation is the ability to simulate a moving boundary at the downstream end, i.e. a delta prograding into standing water. This is important because this is one mechanism that leads to a concave upward profile and downstream fining. Other mechanisms studied through a series of model simulations include sea-level rise and tectonic subsidence. Various rates of sea-level rise (typical of the late Holocene) and tectonic subsidence were modeled in order to quantify their effects on the degree of profile concavity and downstream fining, with comparison to observations in several large, sand-bed rivers.
H33A-06 1330h
Assessing Erosion and Deposition from Comparative Analysis of Long Profiles and Probability Distributions of Dune-bed Elevation at the Red Eye Crossing, Mississippi River
The probability distribution of dune-bed elevation relative to mean bed level (Ps) is a key element of new river morphodynamic models and the analysis of the range and shape of Ps curves is a powerful tool for assessing dune geometry and the relative sediment-transport conditions at a given time. In addition, spatial analysis of successive bed profiles in the along-stream direction provides a direct estimate of erosion and deposition over time. In order to assess the sediment transport or storage within the lower Mississippi River, we repetitively surveyed a 4-km-long bed profile at a major crossing over a period of about 9 months, recording bed elevations with a 1MHz transducer and high-precision echo sounder. Bed profiles show multi-scale bedforms, and hence we constructed Ps curves for selected stretches showing dunes of similar range of height and length. We also compared our data with those from the USACE recorded at the same site in 2001. Preliminary results show that in most cases, the Ps curves are quite symmetric relative to mean bed level and show a narrow range of values. This is comparable to previous experimental results at relatively low sediment-transport stage and indicates that dune troughs are not eroding the bed downward significantly. Although the mean bed level varies little between surveys, the Ps curves reveal time variation in the proportion of elevations above or below mean bed level, indicating the migration of channel-scale sandwaves, for which sediment flux can be estimated. Future research will include relating Ps curves to vertical sorting and velocity profiles, and comparing sediment-transport rates estimated from theory and acoustic measurements.
H33A-07 1330h
Source Determination and Residence Time Indications of Suspended Sediment for the Mississippi River
Characterization of suspended particulates in large river systems is important and challenging. The world's 25 largest rivers, in terms of sediment discharge, account for approximately 40% of the fluvial sediments that enter the ocean. Particulates that enter the ocean from rivers are the products of integrated basin-wide processes. This study focuses on the development of a suite of proxies that give insight to the source and system-wide residence time of particulates discharged by rivers to the coastal zone. To completely characterize the suspended sediments in a major river system, regular sampling over longer periods of time is necessary. A river's suspended load is highly variable over a range of time scales; therefore high temporal resolution sampling is required. Continuous, regular sampling incorporates not only seasonal and annual change, but it also allows interpretation for differences on a sub-seasonal scale. Biweekly sampling of the Mississippi River was performed from March 2002 through December 2004. Previous studies have used short-lived radionuclides (7Be, 137Cs, and 210Pb) to determine sediment source and residence time in small agricultural watersheds. Using the same approach in a large river system is more challenging because sources and sinks are more numerous and complex and residence times change from smaller to larger systems. As a result of these complexities, the use of short-lived radioisotopes alone was not completely diagnostic in the Mississippi River system. Trends in 7Be and 137Cs activities appear to be driven by mixing of old and new sediments. Therefore, both source and residence time effects control the observed radioisotope activities. Additional examinations of grain size, 234U/238U isotope activity ratios, and sediment mineralogy were done to better evaluate basin sources. Four up-basin sampling trips were also performed in February, April, July, and November of 2004. The upper Mississippi, Missouri, Ohio, and Arkansas rivers were sampled each time. These end-member samples are essential to determining primary source, residence time, and are used in a semi-quantitative mass balance analysis for the Mississippi River system. When done in conjunction with basin source indicators, the mass balance approach sheds light on how the main stem of the Mississippi River influences particle retention times, long term storage, and particle alterations.
H33A-08 1330h
Comparing the Dissolved and Colloidal Trace Element Chemistry of Two Major Rivers: the Mississippi and Yukon Rivers
We have been examining the temporal variability of dissolved and colloidal trace element chemistry in two contrasting major river systems: the Mississippi and Yukon Rivers. These two systems are similar in that they both have high suspended loads and moderately alkaline pH's. However, they differ greatly in climate and the extent of human engineering. For instance, in the lower Mississippi the effect of the long residence time of water in upstream impoundments would be expected to provide a major signature. In contrast, the Yukon River has faster flow-through and greater inputs of fresh terrestrial organic matter. Observationally, in the lower Mississippi River seasonal variability in many dissolved metals follows a seasonal "redox pump." This pump is caused by inputs from up-stream seasonally stratified lakes and reservoirs as well as by the temperature dependance of in-stream microbial manganese oxidation. In the Yukon River there is additionally evidence of a spring pulse of trace elements associated with early season soil flushing. The spring pulse has been observed in certain headwaters of the Mississippi River system, so it's absence in the lower river may reflect the effects of impoundments in damping out this signal. We also observe some effects of the greater amount of fresh terrestrial DOC in the Yukon which, for example, results in greater amounts of colloidal Fe in the Yukon relative to the Mississippi. This colloidal Fe could represent a significant pool of reactive Fe delivered to the coastal system. Despite these differences, many dissolved trace elements have similar concentration ranges in the two systems.
H33A-09 1330h
Event-based washload transport and sedimentation in and around flood bypasses: Case study from the Sacramento Valley, California
In large river systems, suspended sediment transport and deposition patterns are often affected by channel constraints engineered for flood conveyance or navigation. Such managed channels typically have a limited number of overflow loci through which suspended sediment enters the river's floodplain. Engineered flood bypasses are narrow relic floodplains that are supplied by overflow diversion weirs along managed river channels, and support agriculture and complex aquatic and riparian habitats that are sensitive to the delivery of floods, fine sediment, and adsorbed contaminants. They function as wide, shallow conveyance channels parallel to the main river, and therefore present an opportunity to assess the applicability of existing theory for delivery to and settling of suspended sediment within floodplains. This study is an investigation of hydrograph characteristics, sediment delivery, and sedimentation within the upstream reaches of flood bypasses closest to the weir. We present analysis of hydrologic and sediment records and modeling in the Sacramento River basin. The effects of a single large flood in 1964-1965 were analyzed by documenting hydrograph characteristics, computing event-based sediment discharges and reach erosion/deposition through the bypass system, modeling bypass deposition, and comparing modeled results near the weirs with dated sediment cores. The rapidly rising, slowly declining 1964 flood was generated by storm runoff in the Sierra Nevada. The modeling results indicate: washload discharge through the lower valley 0.5 to 1.7 times long-term annual averages; mainstem reach erosion/deposition 0.5 to 1.25 times annual averages; and centimeter scale deposition in flood bypasses. The results are corroborated by a set of sediment cores extracted from Sacramento Valley bypasses, which were dated with 210Pb geochronology and analyzed for grain size. The modeling and data suggest net sediment accumulation between the channel and flood weirs and in the `hydraulic shadow' of the flood weir, the length of which varies depending on flow and sediment characteristics. Net accumulation in the hydraulic shadow is hypothesized to be associated with infrequent, episodic erosion of stored upland mining legacy sediments. As a result, more frequent, relatively clear-water flooding erodes prior bypass sediment deposits at the downstream end of the hydraulic shadow and propagates upstream toward the weir. Such sediment remobilization and scour events were extensively documented in our cores and have implications for the fate and transport of contaminants such as mercury, left over from decades of foothill mining, and for sediment and contaminant delivery to the Sacramento-San Francisco Bay-Delta. The modeling and field data highlighted shortcomings in conventional theory for event-based sediment concentration profiles and particle settling. These limitations could be addressed with appropriate data collection and model revision to account for the processes of sediment transport over weirs and into flood conveyance channels.