Peter C. Patton

Gully Erosion, Northwestern Colorado: A Threshold Phenomenon

The widespread occurrence of discontinuous gullies in the oil-shale region of northwestern Colorado is of particular concern because of the resulting progressive destruction of the valley floors. Furthermore, the integration of a semi-arid drainage network can cause a rapid increase in the sediment yield of the basin, with subsequent harmful effects downstream. Field work in the Piceance Creek and Yellow Creek drainage basins indicates that these discontinuous gullies developed on oversteepened segments of the valley floors. Although the critical slope of entrenchment is probably related to magnitude of run-off, discharge measurements are not available; therefore, drainage-basin area was selected as the most representative measure of discharge. An inverse relation between drainage-basin area and critical slope of entrenchment applies, and the lower limit of scatter of the data establishes a critical slope-area relation, which can be used to identify potentially unstable valley floors. This relation can help the land manager determine areas of instability where preventive measures can most economically and successfully be undertaken. It is stressed that this particular quantitative relation is applicable only to the Piceance Creek and Yellow Creek drainage basins. In more heterogeneous basins, other variables will need to be included in the analysis; however, the general theory of valley stability will remain applicable.

Ephemeral-stream processes: Implications for studies of quaternary valley fills

Abstract Three unstable ephemeral-stream channels (arroyos), which drain source areas that have high sediment yields ranging from predominantly sand (Arroyo Calabasas) to a mixture of sand, silt, and clay (Sand Creek) to largely silt and clay (Sage Creek), were resurveyed to provide data on the rates and mechanics of erosion and sedimentation processes during periods ranging from 14 to 22 yr. Channel morphology changed significantly. Erosion occurred through nickpoint recession and bank collapse, but erosional reaches are separated by aggrading or stable-channel reaches. In general, sediment that is eroded, as the nickpoint recedes upstream, is trapped in the widened channel downstream. In this manner sediment is transported episodically out of these basins during a series of cut-and-fill cycles. The manner by which the channels aggrade and the morphology of the aggraded stable channels are controlled by the sediment type. The wide and shallow channel of Arroyo Calabasas is filled by vertical accretion of sand-size sediment. The narrow and deep channels of Sage Creek and Sand Creek are created by the lateral accretion of cohesive fine-grained sediment. The channel modification and the cut-and-fill episodes are dependent on high sediment yields, and therefore they are independent of subtle climatic shifts. Cut-and-fill deposits that have been created in this manner should not be equivalent in age from basin to basin, and therefore channel trenching and filling in the semiarid western United States during the Holocene need not be synchronous.

Sawmill Brook: An Example of Rapid Geomorphic Change Related to Urbanization

The watershed of Sawmill Brook is undergoing urbanization that has increased the frequency of bankfull discharge. The increased runoff is causing extensive bank erosion in the main channel that has increased the size of the bed material and the rate of bedload discharge. The increased frequency of moderate floods, the channel widening, and the change in the sediment transport regime is causing a change from a meandering to a braided channel pattern. With continued urbanization of the basin the present disequilibrium of the channel will be enhanced resulting in an unstable channel pattern for Sawmill Brook.

Processes and rates of formation of Holocene alluvial terraces in Harris Wash, Escalante River basin, south-central Utah

Harris Wash, a tributary of the Escalante River in south-central Utah, has two well-preserved alluvial terraces of compound origin in its lower reaches. The alluvial stratigraphy of the deposits that compose these terraces reflects the complex processes of canyon filling and erosion. The compound terraces were formed by general aggradation of the valley, by deposition from large floods that also resulted in alluvial-fill incision, and by aggradation of flood-plain surfaces during periods of base-level stability. The alluvial chronology indicates that Harris Wash was aggrading for most of the Holocene and that only two late Holocene periods of rapid incision, between 2500 and 1900 yr B.P. and between 1000 and 300 yr B.P., are evident from the terrace stratigraphy. Rapid aggradation of flood plains during the past 150 yr produced historic flood-plain deposits 5 m above the elevation of the stream. The historic flood-plain deposits and other similar deposits in other Escalante River tributaries are attributed to increased runoff and erosion related to land-use changes in the basin during this time interval. Older alluvial deposits in Harris Wash cannot be correlated with alluvial sequences in other western tributaries of the Escalante River basin. In these drainages, valley aggradation and incision are apparently more sensitive to intrabasin processes of sediment production and storage and less dependent on regional factors such as climate change.

Bedload-sediment transport through the Connecticut River estuary

The Connecticut River is the third-largest river on the east coast of the United States, and its estuary is microtidal and sand dominated. Discharge of the river annually varies from tens to thousands of m 3 /s, and circulation in the estuary spans the full range of estuarine mixing modes. During half of the year, the estuary is partially mixed with mutually evasive tidal currents established between the flood-dominated channel and ebbdominated shoal margins. The distribution, orientation, and tidal modification of large bedforms in the estuary demonstrate that there are major pathways of bedload-sediment transport through the estuary to Long Island Sound. Calculations of shear stress on the bed, based on current-velocity measurements at 47 stations through full tidal cycles during low-discharge conditions, indicate that most of the estuary is essentially ebb dominated with respect to bedload-sediment transport. Observations of bedform occurrence and orientation during high discharges indicate that bedload transport is ebb oriented throughout the entire estuary. Analysis of dredging records and historic charts of the estuary since 1915 show little net change in bathymetry and suggest that the bed of the estuary may be in equilibrium with its sediment supply. The combined evidence argues that the Connecticut River estuary is not a site of significant bedload storage, and that it is a conduit of sand into Long Island Sound.

Trace Metals and Radionuclides Reveal Sediment Sources and Accumulation Rates in Jordan Cove, Connecticut

Many small estuaries are influenced by flow restrictions resulting from transportation rights-of-way and other causes. The biogeochemical functioning and history of such systems can be evaluated through study of their sediments. Ten long and six short cores were collected from the length of Jordan Cove, Connecticut, a Long Island Sound subestuary, and analyzed for stratigraphy, radionuclides (14C, 210Pb, 226Ra, 137Cs, and 60Co), and metals (Ag, Cd, Cu, Pb, Zn, Fe, and Al). For at least 3,800 yr, rising sea level has gradually inundated Jordan Cove, filling it with mud similar to that currently being deposited there. Long-term sediment accumulation in the cove averaged close to 0.1 cm yr−1 over the last three millennia. Recent sediment accumulation rates decrease inland from 0.84 cm yr−1 to 0.40 cm yr−1, and are slightly faster than relative sea-level rise at this site (0.3 cm yr−1). Similarity of depth distributions of trace metals was used to confirm relative sediment accumulation rates. 60Co and Ag are derived from sources outside the cove and its watershed, presumably the Millstone nuclear power plant and regional contaminated sediments, respectively. The combined data suggest that Long Island Sound is an important source of sediment to the cove; a minor part of total sediment is supplied from the local watershed. Trace metal levels are strongly correlated with Fe but not with either organic matter or Al. Sediment quality has declined in the cove over the past 60 yr, but only slightly. Cu, Pb, and Zn data correlate strongly with Fe but not with either organic matter or aluminum. Ratios of Ag to Fe and to trace metals suggest that Ag in the cove is derived almost entirely from Long Island Sound. This result supports the notion that Fenormalized Ag can serve as a better tracer of some kinds of contamination than more common and abundant metals, like Cu, Pb, and Zn. *** DIRECT SUPPORT *** A01BY085 00008

New evidence for pre-Wisconsin flooding in the channeled scabland of eastern Washington

Flood-gravel deposits capped by loess sequences that display well-developed argillic, calcic, and petrocalcic paleosols indicate pre-Wisconsin catastrophic flooding in the Cheney-Palouse tract of the channeled scabland, eastern Washington. The most complete stratigraphic exposure reveals two flood-gravel units, one of pre-Wisconsin age and the other representing the last major phase of scabland flooding (late Wisconsin). The two gravel units are separated by three loess units. Each period of loess deposition was followed by a soil-forming interval. The older of the two flood-gravel units contains cobbles of an early pre-Palouse Formation (that is, pre-Bull Lake) loess. It is capped by a loess unit displaying superimposed argillic and petrocalcic soil horizons. Above the petrocalcic horizon are two younger layers of loess (the Palouse Formation), each of which displays a paleosol having a weakly developed argillic B horizon and a calcic C horizon. These units are overlain by gravel from the last major phase of scabland flooding, which is, in turn, overlain by late Wisconsin and Holocene loess. The earliest flood probably carved part of the Cheney-Palouse scabland morphology during a glaciation prior to that responsible for the Palouse Formation.

Sediment storage and terrace formation in Coyote Gulch basin, south-central Utah

Late Holocene terraces in the Coyote Gulch basin, south-central Utah, were formed by aggradation caused by the rapid influx of alluvium produced from landslides on the Straight Cliffs. The main channel of Coyote Gulch was the primary conduit for this sediment and in places aggraded 20 m, burying the preexisting bedrock canyon. This aggradation began about 2300 B.P. and lasted for about 1300 yr. Aggradation in the main stem created a higher base level and reduced the valley gradient in a major tributary, Dry Fork Coyote Gulch, which then partially filled with fine-grained sediment. The major period of aggradation in the Dry Fork was not continuous but was interrupted by incision, presumably when a higher gradient sand-transporting stream was established across this fill. Within the study area more than 25 × 10 6 m 3 of sediment was stored during this aggradational period. Beginning about 900 B.P. this alluvium was incised, creating two terraces in upper Coyote Gulch and as many as three terraces in Dry Fork. Increased sediment production from the erosion of the Dry Fork alluvium caused further deposition downstream in Coyote Gulch, which was eventually incised to form a single terrace. Therefore, rapid sediment loading produced different fluvial responses dependent on the location of a channel segment in the drainage network. Stratigraphic studies can provide insight into the magnitude and rate of sediment storage in stream valleys as well as the timing and processes by which sediment is ultimately transported out of a basin.

Response of the Connecticut River estuary to late Holocene sea level rise

Abstract The morphology of the modern Connecticut River estuary has been inherited from an earlier tidal river. The Holocene evolution of the estuary has been dictated by the relative submergence of the coastline and the sediment trapping efficiency of the estuary and tidal river system. The mid-Holocene tidal river system extended to the modern mouth of the estuary and subaerial floodplains stretched across the modern submerged shoals of the estuary. Submergence in excess of the long-term sedimentation rate converted these floodplains into open backwater coves beginning about 4000 yr BP in the southern estuary and about 2700 yr BP in the upper estuary and it also initiated estuarine circulation in the tidal river. Approximately 1700 yr BP an abrupt decrease in the submergence rate caused freshwater marshes to prograde across the coves and across the remaining floodplain surfaces. At 1000 yr BP salt marsh peat at the mouth of the estuary indicates estuarine circulation similar to that of t he modern estuary. The marshes in the northern estuary have supported freshwater vegetation since their inception. Thus the continuum of modern environments present along the length of the estuary models the evolution of a single reach of the estuary through time. Continued submergence will increase the tidal prism and intensify estuarine circulation. This effect will be regulated by the large excess sediment supply available to fill the volume of the estuary created by relative sea level rise.