Chester C. Watson

Restoration of the Mississippi Delta: Lessons from Hurricanes Katrina and Rita

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

Logistic analysis of channel pattern thresholds: meandering, braiding, and incising

A large and geographically diverse data set consisting of meandering, braiding, incising, and post-incision equilibrium streams was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel pattern and instability. An energy-based index was developed for estimating the risk of channel instability associated with specific stream power relative to sedimentary characteristics. The strong significance of the 74 statistical models examined suggests that logistic regression analysis is an appropriate and effective technique for associating basic hydraulic data with various channel forms. The probabilistic diagrams resulting from these analyses depict a more realistic assessment of the uncertainty associated with previously identified thresholds of channel form and instability and provide a means of gauging channel sensitivity to changes in controlling variables.

Recent morphological evolution of the Lower Mississippi River

Abstract This study documents slope and stream power changes in the Lower Mississippi River during the pre-cutoff (1880s–1930s), and post-cutoff (1943–1992) periods. The study reach extends from New Madrid, MO, to Natchez, MS, a distance of about 900 km. Analyses for six major reaches and 13 sub-reaches for the pre- and post-cutoff periods indicate that the river presently has a much larger slope and stream power than prior to the cutoffs. The largest increases have occurred between Fulton, TN, and Lake Providence, LA, where slope and stream power increases range from about 27% to 36% and 20% to 38%, respectively. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. Previous investigations have shown that no coarsening of the bed material has occurred since 1932, and that the bed material may actually be somewhat finer overall. As the Lower Mississippi River is not a sediment-starved system, an increase in stream power with no change in D50 would be expected to be offset by an increase in the bed material load as the river adjusts towards equilibrium. Previous investigators have inferred a reduction in the sediment loads on the Mississippi River this century based on analyses of total measured suspended loads. However, these results should be viewed as primarily representing the changes in wash load and should not be taken to imply that bed material loads have also decreased. Therefore, the bed material loads in the study reach should be greater than in the pre-cutoff period. Excess stream power in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These elevated sediment loads play a key role in driving morphological adjustments towards equilibrium in the post-cutoff channel. The stability status of the channel in the study reach currently ranges from dynamic equilibrium in the farthest upstream reaches through severe degradation to dynamic equilibrium in the middle reaches, and aggradation in the lowest reaches. These evolutionary trends cannot be explained by consideration of changes in slope and stream power alone. Changes in the incoming bed material load to each reach generated by upstream channel evolution must also be considered.

Stage adjustment in the lower Mississippi River, USA

This study documents the stage adjustments in the Lower Mississippi River during the pre-cut-off (1880s–1930s), and post-cut-off (1943–1994) periods. The study reach extends from Columbus, Kentucky, just downstream of Cairo, Illinois, to Natchez, Mississippi, a distance of about 970 km. The analysis shows that the majority of the pre-cut-off study reach was not undergoing any significant system instability such as channel aggradation or degradation, and, therefore can be considered to have been in a state of dynamic equilibrium during this period. However, the analysis did show that the upper portion of the study reach in the vicinity of Columbus was undergoing a significant aggradational trend during this period. Specific gauge records and peak stage–peak discharge plots for the time period 1950–1994 were analysed to document stage adjustments and to divide the river into the following seven reaches based on observed stability: Columbus to New Madrid (dynamic equilibrium); New Madrid to Fulton (transitional/dynamic equilibrium); Fulton to Sunflower (degradational); Sunflower to Rosedale (transitional); Rosedale to Lake Providence (dynamic equilibrium); Lake Providence to Vicksburg (transitional); and Vicksburg to Natchez (aggradational). Thus, the entire Mississippi River, between Natchez and Columbus is responding in a manner similar to the response of a stream to a single cut-off as described by Lane (1947). Recognition of this evolutionary trend is a first step in developing a comprehensive understanding of this complex system, and will help the engineers and scientists of the US Army Corps of Engineers to develop management strategies for the Mississippi River in the long and short term.

Observed Thresholds of Stream Ecosystem Degradation in Urbanizing Areas: A Process-Based Geomorphic View

Recent studies suggest that channel instability and aquatic ecosystem degradation occur at low levels of watershed imperviousness in humid regions of the U.S. In an effort to provide a more process-based explanation of observed thresholds of aquatic ecosystem degradation in urbanizing areas, U.S. Geological Survey (USGS) gauging data and simple hydrologic techniques were used to examine likely increases in discharge associated with varying levels of watershed imperviousness. The probable increases in these flow parameters for an annual flooding event were compared with analyses of a data set describing 270 stable and unstable channels from diverse geographic regions. Results suggest that a 10 - 20% increase in watershed imperviousness can result in a level of specific stream power that exceeds that corresponding to a high risk of instability for streams poised near a geomorphic threshold. Although risk of channel instability may be associated with different levels of watershed imperviousness, factors such as specific stream power relative to sediment caliber and channel entrenchment provide a more direct assessment of channel sensitivity to urbanization. Linking potential increases in specific stream power with the risk of channel instability establishes a more process-based connection between imperviousness and observed thresholds of aquatic ecosystem degradation.

Modeling Package for Assessing the Potential Effects of Hydrologic Change on Stream Form and Integrity

Existing models can be used to assess the potential hydrologic effects of land-use change, but practical tools for translating these results into predictions regarding channel stability and effects on stream biota are currently unavailable to local planners. To improve watershed management in the context of changing land uses, we are developing a flexible, changeable package of small mechanistic models, statistical models, and expert scientific judgment to provide estimates of long-term changes in stream erosion potential, channel morphology, and instream disturbance regime. We are focusing primarily on the development of a Visual Basic / Excel package of stream / land-use management modules that are designed to operate with either continuous or single-event hydrologic input. Based on input channel geometry and flow series, the various modules provide users with estimates of the following characteristics for pre- and post-land use change conditions: (1) the temporal distribution of shear stress, specific stream power, and potential mobility of various particle sizes; (2) potential changes in channel cross sections as a result of altered flow and sedimentation regimes; (3) frequency, depth, and duration of scour; (4) effective discharge / sediment yield; (5) sediment transport capacity based on selected transport equations; and (6) analytical computation of stable channel dimensions. An attractive feature of this approach for stormwater management is a set of user-friendly tools to examine timeintegrated sediment transport and scour characteristics across a range of flows and time periods associated with varying stormwater mitigation schemes. Ultimately, these modules will give end users a suite of tools to compare the erosive potential of hydrographs, to depict channel changes that might result from different land-use management scenarios, and to improve interpretation of biomonitoring information through consistent quantification of stream disturbance regimes.

Mississippi River Streamflow Measurement Techniques at St. Louis, Missouri

AbstractStreamflow measurement techniques of the Mississippi River at St. Louis have changed through time (1866–present). In addition to different methods used for discrete streamflow measurements, the density and range of discrete measurements used to define the rating curve (stage versus streamflow) have also changed. Several authors have utilized published water surface elevation (stage) and streamflow data to assess changes in the rating curve, which may be attributed to be caused by flood control and/or navigation structures. The purpose of this paper is to provide a thorough review of the available flow measurement data and techniques and to assess how a strict awareness of the limitations of the data may affect previous analyses. It is concluded that the pre-1930s discrete streamflow measurement data are not of sufficient accuracy to be compared with modern streamflow values in establishing long-term trends of river behavior.

Assessment of Phosphorus Concentrations in Bank Sediments of Long Creek, MS

Excessive sedimentation degrades the quality of aquatic habitat, diminishes watershed functions and services, and limits the aesthetic, social, and economic value of aquatic ecosystems. Sedimentation processes are closely linked with the fate and transport of phosphorus (P) in streams. Stream bank and bed erosion are a major source of particulate phosphorus in channelized streams. A total phosphorus concentration in the range of 200 -300 mg/kg appears to be a reasonable overall value for bank material that is likely to erode in the Long Creek watershed. This value is reasonably consistent across all sampled sites and the content of soil in other areas. Since phosphorus is the limiting nutrient in most freshwater systems, 0.4 lbs of total P per ton of bank material eroded (200 mg/kg) could be significant in terms of affecting water quality in downstream systems. This paper documents sampling and testing techniques, and the spatial distribution of concentrations within the bed and banks. Sediment Transport and Phosphorus Dynamics in Streams Nutrient enrichment of streams, lakes, and estuaries has become one of the most pressing environmental problems facing society today. Riparian zones and vegetative buffers have been shown to effectively reduce the delivery of critical pollutants such as phosphorus, sediment, and nitrogen to surface waters in a wide variety of riverine environments. Like riparian processes, channel geomorphology has a critical role in controlling nutrient delivery and cycling processes in stream ecosystems (Allan, 1995). Since inorganic phosphorus is typically the element that limits primary production in freshwater streams, rivers, lakes, and reservoirs (Schindler, 1977; Wetzel, 1983; Hecky and Kilham, 1988), determining the role of riparian zones and channel processes in controlling the response of watersheds to phosphorus loading is a critical research need. Excessive sedimentation degrades the quality of aquatic habitat, diminishes watershed functions and services, and limits the aesthetic, social, and economic value of aquatic ecosystems. The adverse impacts of excessive erosion include reduced habitat for fish and

System Level Analysis of Watershed Instability in the Yalobusha Basin, Mississippi

Channel modifications during the 1960s have resulted in watershed instability in the Yalobusha River Basin, Mississippi. A comprehensive analysis of the watershed was conducted to provide a basis for design of diverse watershed rehabilitation practices. Proposed actions range from instream grade control to upland best management practices. Results of the analyses demonstrate a coupled geomorphic-hydraulic engineering approach can be utilized.

Stable Channel Design For Mobile Gravel Bed Rivers

An analytical method for predicting stable channel morphology of gravel bed rivers was developed in a menu-driven computer interface that facilitates input and output for design. The computer program was tested through application to a stable gravel bed river. Evaluation of the results was conducted by comparing the design to existing reach morphology and results from other methods. A sensitivity analysis was also conducted to provide insight for practical applications.