Changes In Sediment Load Research Articles

Nile sediment dispersal altered by the Aswan High Dam: The kaolinite trace

Proportions of kaolinite have increased substantially in lower River Nile clay mineral assemblages since emplacement in 1964 of the High Dam at Aswan, Egypt. Monitoring this recent increase can be used to help gauge sediment load changes induced by the High Dam in the lower River Nile, between the dam and Nile delta and, eventually, offshore in the Mediterranean's Levantine Basin. High kaolinite values (to 50%) are recorded north of the new Nile delta presently forming in Lake Nasser, and are even higher below the High Dam (to 70%). Kaolinite is introduced to Lake Nasser from adjacent desert terrains largely from wind transported material and wave erosion of lake margins. As the major clay component of the suspended sediment load in lake waters, this mineral is released through the dam. Additional kaolinite enters the Nile system just below the High Dam from late Pleistocene and older sections scoured by swift, turbulent Nile waters flowing northward of the dam. During the past 31 years, sediment containing higher concentrations of kaolinite (~30%) has migrated northward at a rate of ~ 10 km/year, and presently extends to Qena ~ 350 km north of the dam; this stretch defines the River Nile sector most altered since closure of the High Dam. It is predicted that enhanced proportions of kaolinite will be recorded at Cairo and the southern Nile delta within 50 years, and at the Mediterranean coast by the end of the next century.

POSTFIRE SUSPENDED SEDIMENT FROM YELLOWSTONE NATIONAL PARK, WYOMING1

ABSTRACT: Wildfires in 1988 burned over 2000 square miles of the greater Yellowstone area in Montana and Wyoming in the largest fires in the history of Yellowstone National Park (YNP). A four‐year postfire study to estimate fire‐related changes in suspended sediment transport on the Yellowstone River and its principal tributary in YNP, the Lamar River, benefitted from a recently completed three‐year prefire baseline study. Both studies took daily depth‐integrated samples from April through September. Fire‐related changes in suspended sediment were distinguished from natural climatic variations by two methods: comparison of forecast postfire sediment loads estimated with prefire sediment‐rating equations to measured postfire loads; and by postfire changes in suspended sediment load expressed per unit volume runoff. Both methods indicated postfire sediment increases that varied according to season. The higher elevation Lamar River basin had little postfire increase in spring snowmelt season sediment but large increases in summer sediment load. The Yellowstone River had postfire increases in sediment load for the spring but did not reflect the large summer increases of its upstream tributary. The reasons for the difference in postfire snowmelt sediment response are unclear but may relate to basin elevation differences, the effects of unburned watersheds, and cooler postfire springs. The few high streamflow snowmelt events in the postfire period mitigated postfire sediment increases.

Erosion and sediment yield in a changing environment

Abstract Existing assessments of patterns of global sediment flux from the land to the oceans and of global patterns of sediment yield have tended to treat the global denudational system as a static system. There is, however, increasing evidence that the sediment loads of the world’s rivers have changed significantly as a result of human activity and interference. As well as being of scientific interest, such changes have important environmental and economic implications and should be included in current concerns for the impact of global change on the Earth’s environment. Lack of reliable long-term records precludes a detailed assessment of longer-term and recent changes in suspended sediment transport by world rivers, but the available information can be supplemented by evidence from several other sources, which include the long-term geological perspective, lake sediment records, catchment experiments and space-time substitution; all of which provide valuable information concerning the sensitivity of river sediment loads to environmental change and the magnitude of the changes involved. There is evidence of significant increases and decreases in sediment yields in many areas of the world. Any attempt to relate such changes in sediment loads to environmental change within the upstream drainage basin must, however, take account of the complexity of the sediment delivery system.

River adjustment to changes in sediment load: The effects of tin mining on the Ringarooma River, Tasmania, 1875–1984

AbstractThe mining of alluvial tin in the Ringarooma basin began in 1875, reached a peak in 1900–20, and had virtually ceased by 1982. During that time 40 million m3 of mining waste were supplied to the main river, quickly replacing the natural bed material and requiring major adjustments to the channel.Based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load, the river's transport history is reconstructed using a mass‐conservation model. Because of the lengthy time period (110 years) and river distance (75 km) involved, the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream. Peak storage is predicted in that part of the river where braiding and anastomosis are best developed.Aggradation was most rapid in the upper reaches close to major supply points, becoming slower and later with distance downstream. Channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common. Bridges had frequently to be replaced. While bed levels were still rising in lower reaches, degradation began in upper ones, notably after 1950, and by 1984 had progressed downriver over 30 km. Rates of incision reached 0·5 m yr−1, especially in the early 1970s when record high flows occurred. As a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions. Also a narrower unbraided channel has developed. The river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris.

Historical development of the saltmarsh at Wells, Maine

AbstractComparisons of maps and aerial photographs dating from the late 1700s to the present document the recent development of an 8 km2 saltmarsh that is situated behind a barrier spit in southern Maine. Tidal channels that were relatively narrow in 1794 became wider by 1872. The reduction of marsh bordering tidal channels is interpreted as evidence that marsh accretion could not keep pace with rising sea‐level. This suggests that the rate of sea‐level rise had increased, although a change in discharge or sediment load caused by extensive settlement and land clearance may also have been involved.Meander patterns of the tidal streams changed considerably throughout the time period covered by the maps, demonstrating that the streams of this marsh are more dynamic than some others that have been widely reported in the literature. These differences in stream dynamics are probably related to the differences in sedimentological structure of the marshes.Between 1872 and 1956 the barrier spit eroded on its inside (shoreward) edge, probably in response to the construction of riprapping and houses along the spit, and the subsequent reduction of overwash and aeolian transport of sediment.Modification of the tidal inlet and adjacent marsh during the 1960s, including jetty construction, dredging, and filling of portions of the marsh surface, affected the marsh only locally. One tidal stream has been migrating rapidly apparently in response to compaction of peat by dredge spoils and consequent local disruption of the marsh hydrology. Except for this migration, erosion of the marsh edge occurred immediately after the inlet modifications; planimetric changes in the marsh and its streams have been minor since then.

The b-f-m Diagram for Downstream Hydraulic Geometry

ABSTRACTThe b-f-m diagram can be used to interpret the downstream hydraulic geometry of rivers. Six lines representing thresholds of adjustment to downstream changes in discharge and sediment load ...

River metamorphosis and environmental change in the Macdonald Valley, New South Wales, since 1949

A significant metamorphosis of channel form occurred on the lower Macdonald River between 1949 and 1955; width and width‐depth ratio increased and depth, sinuosity and weighted mean per cent silt‐clay in the channel perimeter decreased. The channel has remained unstable since 1955, exhibiting frequent variations in bed elevation and a recent period of minor channel contraction followed by slight enlargement. Since 1946 there has been an increase in summer and annual rainfall and an abrupt, upward shift of the annual series flood frequency curve. As a result an increased proportion of the total sediment load of the river is now being transported as bed‐material load. The change in sediment load is a result of greater stream competence and the consequent reworking of sandy sediment temporarily stored within the channel as benches. The observed channel changes are a quasi‐adjustment to the increased flood and bed‐material load discharges.

Sediment removal during an extreme summer storm: Muskwa River, northeastern British Columbia

Extreme summer storms in northwestern Canada can produce large changes in river channels and sediment loads in a short time. These infrequent events cause effects downstream that persist over several years. Photogrammetric measurement was made of sediment removed during a 36-h storm flood in June, 1977, on the Muskwa River near its confluence with the Fort Nelson River. The approximate volume of material removed was 1 398 000 m3. The sediment load added to the Fort Nelson River was between 2.3 and 2.4 million tonnes. Photogrammetric measurement techniques, combined with field samples, can provide adequate information on sediment removed during severe storms. Keywords: erosion, sediment, photogrammetry, storms, floods.

Effects of suspended sediment on fish eggs and larvae: A laboratory assessment

Eggs and larvae of 6 species of anadromous and estuarine fish indigenous to the Chesapeake Bay were exposed to concentrations of suspended sediment ranging from a few mg l −1 to 1000 mg l −1 to determine the effects of different concentrations on hatching success and short term survival. The egg experiments indicated that concentrations of up to 1000 mg l −1 did not significantly affect the hatching success of yellow perch, blueback herring, alewife or American shad eggs. Concentrations of 1000 mg l −1 significantly reduced ( P < 0·05) the hatching success of white perch and striped bass, but lower concentrations did not. Experiments with larvae indicated that concentrations of ⩾500 mg l −1 significantly reduced ( P < 0·05) the survival of striped bass and yellow perch larvae exposed for 48–96 h. American shad larvae appeared to be less tolerant than the other two species tested. Concentrations ⩾100 mg l −1 significantly reduced the survival of shad larvae continuously exposed for 96 h. The significance of these results are discussed relative to natural and man-induced changes in sediment loading of estuaries.

Adjustment to Quaternary Climatic Change by the Colorado River in Central Texas

Geomorphic mapping of flood plain and channel morphology in the Quaternary alluvial valley of the Colorado River revealed ten cross-cutting assemblages of fluvial channel patterns and associated sedimentary structures and textures. Quantitative studies of the modern and ancient patterns between Austin and La Grange, Texas, indicate systematic changes in channel width, meander wavelength, sinuosity, and sediment type through time. The changes appear to be cyclic, although the amplitudes of the cycles vary. Sufficient morphology is preserved to utilize empirical correlations of meander wavelength to discharge as an aid in paleohydrologic interpretation. However, the influences of sediment load and flood magnitude-frequency changes preclude quantitative speculation. Data from fluvial sediments, relict paleosols, and paleobiological studies support the hypothesis that Quarternary climate in central Texas alternated between relatively arid and humid intervals. Humid phases were characterized by fine-grained lo...

Experimental Study of Channel Patterns

A series of experiments was performed in a large flume to determine the effect of slope and sediment load on channel patterns. Sediment loads and slopes were closely related, and as slope and sediment loads increased, threshold values of these variables were encountered, at which channel patterns altered significantly. At a very low slope and sediment load, the channels remained straight, but at a discharge of 0.15 cfs, a meandering-thalweg channel formed at slopes greater than 0.002. With increased slope and sediment loads, thalweg sinuosity increased to a maximum of 1.25. At slopes greater than 0.016, a braided channel formed. The model channels responded to increased sediment loads by maintaining steeper gradients and by major channel pattern changes, but at very gentle slopes and at steep slopes, the channel could not be forced to develop a meandering thalweg. These experiments suggest that landforms may not always respond progressively to altered conditions. Rather, dramatic morphologic changes can occur abruptly when critical erosional and (or) depositional threshold values are exceeded. The meandering-thalweg channel was not a meandering channel. A truly meandering channel with a sinuosity of 1.3 formed when a suspended-sediment load (3 percent concentrations of kaolinite) was introduced into the flow. The clay stabilized the alternate bars, and scour and deepening of the thalweg resulted. This in turn lowered the water level at constant discharge, and the alternate bars emerged o t form point bars. A meandering-thalweg channel was thus converted to a meandering channel by the type of sediment load change that has accompanied climatic and hydrologic changes of the recent geologic past.