Suzanne M. Orbock Miller

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.

Beach Cusp Destruction, Formation, and Evolution during and Subsequent to an Extratropical Storm, Duck, North Carolina

Many studies have debated whether beach cusps are erosional or depositional features. The April 12-14, 1988, extratropical storm provided an opportunity to view the direct effects of one of the largest storms of the past decade upon beach sedimentology and morphology on barrier islands near Duck, North Carolina. Prior to the storm, the beach at Duck was characterized by a well-defined pattern of beach cusps with horn-to-horn spacings averaging 35 m. Storm-induced alterations were dominated by an initial period of beach erosion that remobilized the upper 30 to 50 cm of beach sediment, followed by aggradation. Net aggradation was most prominent along the middle beachface and within the pre-storm cusp bays. These morphologic adjustments resulted in the destruction of cusps, which were replaced with a post-storm planar beachface composed of horizontally bedded fine- to coarse-grained sediments. Within 24 hrs of storm subsidence, new beach cusps formed sequentially along the coast in the direction of longshore transport. Initial cusp formation resulted from beach erosion and the creation of bays in the planar storm-beach surface at positions of preferential post-storm runup. The initial cusp horns were composed of truncated horizontal beds of the planar beach accreted during the storm. After their formation, the cusps sequentially migrated downdrift. Migrating horns were composed of a coarse-grained sediment wedge that thickened toward horn crests, suggesting formation by deposition. It is concluded from these observations that beach cusps are both erosional and depositional in nature.

Application of Geochemical Tracers to Fluvial Sediment

Use of geochemical fingerprinting methods to determine sediment provenance has progressively increased since the late 1990s, and is now considered by many investigators as the method of choice to quantify sediment source contributions at the catchment scale. Application of geochemical fingerprinting largely rests on four factors: (1) the inability of other techniques (e.g., sediment load monitoring, photogrammetric methods, and mathematical modeling approaches) to effectively determine sediment provenance at the required spatial scales, (2) improvements in analytical methods that allow for the analysis of large numbers of samples for a wide range of elements, (3) the modification of the utilized statistical methods (e.g., inverse/unmixing models) to more effectively account for uncertainty in the modeled results, and (4) the ability to apply themethods to historic sedimentary deposits retrospectively to determine changes in sediment provenance at a site through time. In this chapter, we focus on the application of geochemical fingerprinting to contemporary river sediments as well as alluvial deposits that are less than about 150 years old. Our intent is not simply to summarize the voluminous and growing body of literature on the subject, but to document the strengths, weaknesses, and uncertainty inherent in the approach.

Stable ‘Non-Traditional’ Isotopes

Recent advances in analytical capabilities, particularly the MC-ICP-MS, have allowed for a precise determination of a wide range of stable isotopes in geological and biological materials that could not be assessed prior to the 1990s. As a result, research into the use of these ‘non-traditional’ isotopes (or ‘non-CHONS’) as tracers of both elemental sources and biogeochemical processes has been increasing at an exponential rate. While their utilization as a tracer of contaminated sediments in the near surface environment is often complicated by multiple physical and biological fractionation processes, there is increasing evidence to suggest that they may be effectively used as tracers in aquatic environments. In this chapter, we examine the potential use of four stable metal isotopes (Zn, Cd, Cu, and Hg) that appear on the basis of the limited studies conducted to date to have the potential to track sediment-associated trace metals in rivers.