Dale F. Ritter

Competence of Rivers To Transport Coarse Bedload Material

Shear-stress analyses provide easily determined estimates of the competence of rivers to transport coarse bedload material. Relevant data from diverse geological and engineering reports are integrated, and a good correlation between competent particle size and shear stress results when data points are derived in a uniform way. Because flow mechanics differ in contrasting fluvial environments, the shear stresses needed to entrain large sediment may diverge considerably from values based on theoretical grounds. In shallow rivers, entrainment may occur at lower shear stresses than predicted by the Shields theory because hydrodynamic lift and bank caving provide additional transporting force. In very deep flows, the shear stresses needed to initiate particle movement are greater than theoretical values. Potential use of the empirical relationship between particle size and shear stress is limited by the following considerations: (1) random nature of turbulent life forces, (2) problems in interpreting the significance of the sedimentary deposit, (3) sediment finer than 5 cm in diameter is not considered, and (4) the analysis ignores special considerations of sediment packing, shape, and grain-size distributions.

Overbank Sedimentation in the Delaware River Valley during the Last 6000 Years

A thick sequence of floodplain sediments has accumulated in the Delaware River Valley by the process of overbank deposition. Textures in the sediments indicate that the sequence contains no point-bar deposits and is unbroken by periods of erosion. Fourteen radiocarbon dates show that deposition began at least 6000 years ago and has continued to the present. Because the Delaware River shifts its position laterally at a very slow rate, overbank deposition becomes dominant in the construction of its floodplain.

Fluvial responses to land-use changes and climatic variations within the Drury Creek watershed, southern Illinois

Abstract Fluvial responses to climatic variation and Anglo-American settlement were documented for the Drury Creek watershed, southern Illinois by examining stratigraphic, geomorphic, climatic, and historical data. Regional analyses of long-term precipitation records document a period of decreasing mean annual precipitation from 1904 to about 1945, and an increasing trend in annual precipitation from 1952 to the present. The period between 1945 and 1951 experienced a large number of intense storms that resulted in high annual precipitation totals. Statistical relationships illustrate that changes in precipitation totals are transferred to the hydrologic system as fluctuations in stream discharge. Historical records of southern Illinois show that a maximum period of settlement and deforestation occurred between the 1860s and 1920s. This era ended in the 1940s when large tracts of land were revegetated in an attempt to curtail erosion which had caused extensive upland degradation. In response to hillslope erosion at least two meters of fine-grained sediments were deposited on valley floors. Average sedimentation rates, determined using decdrochronologic techniques, are estimated to be 2.11 cm/yr for the period between 1890 and 1988; rates that are 1 to 2 orders of magnitude greater than pre-settlement values calculated for other areas of the midwest. However, botanical data suggest that aggradation was episodic, possibly occurring during three periods characterized by greater annual precipitation. Since the 1940s, sedimentation rates have declined. Reduced rates of sedimentation are related to an episode of channel entrenchment that reduced overbank flooding. Entrenchment coincided with a period of: (1) reduced sediment yields associated with watershed revegetation and the introduction of soil conservation practices, and (2) intense storm activity that resulted in long periods of high discharge. As a result of channel incision and hillslope erosion, newly exposed bedrock in upstream areas currently provides a source of gravel load to the channels. The distribution of coarse bedload material along tributary streams combined with downstream decreases in width:depth ratios and tractive force estimates suggest that channels in the Drury Creek watershed are slowly adjusting their configuration to transport coarse bedload material. The fluvial response to the increased influx of coarse sediment began more than 45 years ago and continues today.

Role of tree dams in the construction of pseudo-terraces and variable geomorphic response to floods in Little River valley, Virginia

Geomorphic response to a 1985 flood in Little River valley, northern Virginia, was different in magnitude and style from the largest historic flood in the same valley in 1949. The primary geomorphic activity during the 1985 flood was severe bank erosion and channel-gravel deposition rather than the debris flow and avalanching of the 1949 event. An unusual and widespread phenomenon of the recent flood was that large trees eroded and transported by the floodwater were aligned parallel to the river banks and, at isolated sites, were braced and stacked against trees still standing on the floodplain. Lateral barriers or dams created from these displaced trees allowed the channel to be locally aggraded above the level of the flood-plain. In these reaches, little, if any, river gravel was deposited on the floodplain, even though the adjacent channel floor was raised well above that surface. The river has now shifted around the filled segments, leaving flat, isolated surfaces, underlain by channel gravel, standing above the level of the modern floodplain. These features may be mistaken for terraces alter they become vegetated and the trees bracing the gravels decay. Interpreting these surfaces to be terrace remnants would lead to a faulty reconstruction of geomorphic history in the Little River valley and other valleys where floodplain morphology is controlled by infrequent flood events.

The disruption of Grassy Creek: implications concerning catastrophic events and thresholds

Two catastrophic events, occurring simultaneously in the valley of Little Grassy Creek, IL, allow for an examination of the threshold concept in geomorphology. Movement of debris associated with failure and sliding of valley-side material, caused damming and avulsion of Little Grassy Creek. Slope and river disruptions, both severe in character, were linked because the effect of one event (slope failure) was the cause of the second event (fluvial avulsion). The slope failure represents a true threshold-crossing event because the results are irreversible on a graded-time scale. In contrast, the fluvial disruption was not a threshold crossing, although the event was catastrophic and short-term instability occurred. In the fluvial case, a new channel developed, and the re-establishment of equilibrium, as estimated by channel characteristics, occurred within 10 years. The river system functions as it did before the slope failure/avulsion, though the channel reach is now in a different location. Criteria needed to employ thresholds to explain geomorphic events are suggested, and a definition of thresholds as time-dependent phenomena is presented as a means of reducing confusion over the use of the threshold concept.

Geomorphic response to minor cyclic climate changes, San Diego County, California

Abstract Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region. Using aerial photographs and field investigations we found little response to the 1978–1983 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.

Is Quaternary geology ready for the future

Abstract Armed with a better understanding of process and an array of developing dating techniques, Quaternary geology is poised to achieve greater recognition in the general scientific community. This recognition however, will require some thought adjustment. Quaternary geologists will have to convince government, industry and a variety of scientific groups that they possess unique training and expertise that is needed as part of the thrust to fully understand and/or resolve major scientific problems. Therefore, future research and education efforts should not focus on developing a rigidly defined identity within geoscience, but instead should seek ways to be integrated with interdisciplinary teams that will investigate complex environmental and climate change problems. Such a scenaria creates and enermous dilemma for Quaternary geologists because they will derive greater intellectual stimulation from scientists working in discplines other than geology, and their scientific collaboratiors will most likely not be their academic colleagues. This outward expansion of our scientific network will require the development of interdsciplinary research collaboration and/or degree-granting programs at the graduate level. To accomplish such goals, universities must resist “turf protection”, and funding agencies muts become more efficient at facilitating interdisciplinary research.

Cobble imbrication as a sensitive indicator of subtle local changes in river flow direction

Imbrication of cobbles in Pleistocene terrace gravels along the Shoshone River near Powell, Wyoming, shows significant changes in orientation from a lower set to an upper set within a gravel layer. The lower gravels show a mean orientation of 126°, with spreads from s = 38° to s = 56°. The mean orientation of the upper gravels is 070°, with spreads from s = 19° to s = 26°. Change from the lower to the upper deposit is abrupt, with no noticeable variability near the contact. In fact, the contact can easily be overlooked without close scrutiny of the cobble fabric. This suggests that cobble imbrication may be a sensitive indicator of subtle changes in the flow direction of a braided stream. Because the earlier fabric was not disturbed by deposition of the younger unit, there is every reason to believe that such fabrics can and will be preserved even in the consolidated rock record and can be used for paleoenvironmental interpretations.

Complex river terrace development in the Nenana Valley near Healy, Alaska

A complex sequence of river terraces in the Nenana Valley was examined to determine whether the surfaces were depositional or erosional in origin. Of prime concern was the identification of outwash derived from the Healy (early Wisconsin) and Riley Creek I (late Wisconsin) glaciations. Two terraces were formed during the Healy glacial cycle. The higher level represents the depositional surface of the Healy outwash. The lower level was formed during recession of the Healy ice when approximately 18 metres of that outwash was removed. The erosion was initiated when glacial Lake Moody, dammed behind the Healy moraine, spilled over the drainage divide and began excavation of the Nenana gorge. The surface of Riley Creek [] outwash has been offset by faulting near the gorge mouth. Isolated remnants of that surface stand at different elevations, complicating the correlation of downvalley outwash with the Riley Creek I moraine. The evolution of the Nenana gorge is associated with spasmodic development of erosional terraces throughout post-Healy time as the Nenana River attempted to establish a new equilibrium condition. This erosional trend has been interrupted only by out-wash deposition during episodic expansion of the Riley Creek ice.