Evaluation of the method of collecting suspended sediment from large rivers by discharge-weighted pumping and separation by continuous-flow centrifugation

A method for collecting suspended sediment samples has been developed that pumps a discharge-weighted volume of water from fixed depths at four to 40 locations across a river and separates the suspended sediment in the sample using a continuous-flow centrifuge. The efficacy of the method is evaluated by comparing the particle size distributions of sediment collected by the discharge-weighted pumping method with the particle size distributions of sediment collected by depth integration and separated by gravitational settling. The pumping method was found to undersample the suspended sand sized particles (> 63 μm) but to collect a representative sample of the suspended silt and clay sized particles (< 63 μm). The centrifuge separated the silt and clay sized particles (< 63 μm) into three fractions. Based on the average results of processing 17 samples from the Mississippi River and several of its large tributaries in 1990, about 10% of the silt and clay sized material was trapped in a centrifuge bowl-bottom sealing unit containing the nozzle and consisted of mostly medium and coarse silt from 16 to 63 μm. About 74% was retained on a Teflon liner in the centrifuge bowl and consisted of sizes from 0–1 to 63 μm. About 9% was discharged from the centrifuge in the effluent and was finer than 0–1 μm. About 7% was lost during the processes of removing the wet sediment fractions from the centrifuge, drying and weighing. The success of the discharge-weighted pumping method depends on how homogeneously the silt and clay sized particles (< 63 μm) are distributed in the vertical direction in the river. The degree of homogeneity depends on the composition and degree of aggregation of the suspended sediment particles.     

Terrace aggradation during the 1978 flood on Powder River, Montana, USA

Flood processes no longer actively increase the planform area of terraces. Instead, lateral erosion decreases the area. However, infrequent extreme floods continue episodic aggradation of terraces surfaces. We quantify this type of evolution of terraces by an extreme flood in May 1978 on Powder River in southeastern Montana. Within an 89-km study reach of the river, we (1) determine a sediment budget for each geomorphic feature, (2) interpret the stratigraphy of the newly deposited sediment, and (3) discuss the essential role of vegetation in the depositional processes.Peak flood discharge was about 930 m³ s^− 1, which lasted about eight days. During this time, the flood transported 8.2 million tons of sediment into and 4.5 million tons out of the study reach. The masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach. The flood inundated the floodplain and some of the remnants of two terraces along the river. Lateral erosion decreased the planform area of the lower of the two terraces (~ 2.7 m above the riverbed) by 3.2% and that of the higher terrace (~ 3.5 m above the riverbed) by 4.1%. However, overbank aggradation, on average, raised the lower terrace by 0.16 m and the higher terrace by 0.063 m.Vegetation controlled the type, thickness, and stratigraphy of the aggradation on terrace surfaces. Two characteristic overbank deposits were common: coarsening-upward sequences and lee dunes. Grass caused the deposition of the coarsening-upward sequences, which had 0.02 to 0.07 m of mud at the base, and in some cases, the deposits coarsened upwards to coarse sand on the top. Lee dunes, composed of fine and very fine sand, were deposited in the wake zone downstream from the trees. The characteristic morphology of the dunes can be used to estimate some flood variables such as suspended-sediment particle size, minimum depth, and critical shear velocity. Information about depositional processes during extreme floods is rare, and therefore, the results from this study aid in interpreting the record of terrace stratigraphy along other rivers.     

Suspended-sediment and fresh-water discharges in the Ob and Yenisey rivers, 1960–1988

Of the world's great rivers, the Ob and Yenisey rank among the largest suppliers of fresh water and among the smallest suppliers of suspended sediment to the coastal ocean. Sediment in the middle reaches of the rivers is mobilized from bordering terraces and exchanged between channels and flood plains. Sediment in the lower reaches of these great rivers is deposited and stored (permanently, on a millennial time scale) in flood plains. Sediment discharges, already small under natural conditions, are diminished further by large manmade reservoirs that trap significant proportions of the moving solids. The long winter freeze and sudden spring breakup impose a peakedness in seasonal water runoff and sediment discharge that contrasts markedly with that in rivers of the tropics and more temperate climates. Very little sediment from the Ob and Yenisey rivers is being transported to the open waters of the Arctic Ocean under present conditions.     

Revisiting the orogenic energy balance in the western Taiwan orogen with weak faults

Orogens at convergent margins must meet the energetic requirements necessary to lift rocks against gravity, allow for frictional sliding along basal detachments and accommodate internal deformation processes. The combination of critical taper and kinematic wedge theories predicts the partitioning between these energy sinks as a function of both fault and crustal strengths. Integrating contemporary estimates of both crustal pore fluid pressures and the coefficient of friction on major faults, we find that work associated with internal deformation processes is the dominant energy sink in the western Taiwan orogenic wedge. These processes consume 54% of the total work budget, while the dissipation rates associated with frictional sliding on the basal detachment and lifting rocks against gravity are lower, requiring only 11% and 35% respectively. These results suggest a mechanical dichotomy in orogenic wedges where the faulting dominates the kinematic deformation budget, but internal distributed deformation processes dominate the energy budget.     

Biomimetic phosphoryl transfer catalysed by iron(II)-mineral precipitates

Iron(II) minerals have been found to catalyse the formation of pyrophosphate from activated phosphate compounds and inorganic phosphate at near neutral pH in water. Iron(II) phosphate catalyses the formation of pyrophosphate, from either acetyl phosphate or phosphoenolpyruvate as the activated phosphoryl donor, in yields of up to 25% and 14% respectively. Under similar conditions, these minerals also retard the hydrolysis of pyrophosphate. The catalysis of pyrophosphate formation is tolerant of sulfide ions: pyrophosphate being produced from acetyl phosphate in 12% yield in the presence of equimolar amounts of sulfide. These reactions mimic the extant biosynthesis of ATP from acetyl phosphate or phosphoenolpyruvate; they counter the concern that hydrolysis of polyphosphates would out-compete their accumulation under prebiotic conditions; and hence suggest a possible prebiotic route to polyphosphates under conditions that may have occurred on the early earth.     

Solar system observations by the Wisconsin Ultraviolet Photopolarimeter Experiment– III. The first ultraviolet linear spectropolarimetry of the Moon

Portions of the Moon were observed by the Wisconsin Ultraviolet Photopolarimeter Experiment ( WUPPE ) on 1995 March 12, 14 and 17, and represent the first ultraviolet (UV) spectropolarimetric observations of the Moon. The polarimetric observations confirm that a change in the dominant scattering process occurs in the UV, changing from volume scattering in the near-UV to surface scattering in the far-UV. The data are investigated empirically. It is found that Umov's relationship holds when the polarization is perpendicular to the scattering plane. It is also found that the degree of polarization can be modelled by a phase-angle-dependent polarization modified by a wavelength-dependent depolarization factor. The scattering function for each observation is determined.