Robert Dolan

Geomorphology of the Colorado River in the Grand Canyon

Sediment supplied to the Colorado River within the Grand Canyon has been sorted into distinct deposits of three grain size ranges. The major rapids are formed by boulder deposits from side-canyon tributaries. As a result of a fourfold reduction in peak discharge when Glen Canyon Dam was closed in 1963, new fan debris may increase the gradient through some of the rapids by a factor of 1.8. Cobbles and gravel, transported only during flood stages, are preferentially deposited in the wider sections of the river as bars and riffles and are, for the most part, inactive during post-dam discharges. Fine-grain (largely sandy) terraces occur throughout the canyon, especially along the banks of the large reverse eddies above and below the rapids. The lower terraces are being reworked into beach-like shores by diurnally-varying, post-dam discharges. A slight net lateral erosion of the terraces has resulted. Prior to construction of the dam, sandy bed deposits underwent scour averaging about 1 m during spring floods, balanced by deposition from tributary sources during the summer. Downstream from rapids, decreased turbulence due to lower discharges has resulted in deposition averaging 2.2 m on the bed within the upper portions of the canyon. Differences in rock types along the river determine overall channel morphology. Rocks of low resistance result in a wide valley, a meandering channel, and abundant cobble bars and sand terraces. Narrow channels with rapids and deep pools are most frequent within the sections of the canyon where Precambrian crystalline rocks dominate.

A new photogrammetric method for determining shoreline erosion

Abstract In order to systematically measure shoreline erosion and storm surge penetration along extensive reaches of the United States Atlantic coast, a common-scale mapping method was developed using historical aerial photography as the data base. Aerial photography of the southern New Jersey coast covering four decades is used to demonstrate the methodology and to provide long-term baseline information on shoreline dynamics. The data sets include mean erosion rates and variance at 100-m intervals along the coast. Shoreline recession rates along the New Jersey coast are generally less than 1 m/yr. but for several locations rates exceed 5 m/yr., and they vary considerably both within and between the island segments of the New Jersey coast.

Structural Control of the Rapids and Pools of the Colorado River in the Grand Canyon

Most of the major rapids along the 450-kilometer course of the Colorado River in the Grand Canyon are within fracture zones that run perpendicular to the river. Steep tributaries flowing within the zones of bedrock weakness move large debris to the Colorado, forming the rapids. Accelerated flow through the rapids scours the deep pools that are located below them.

Classification of coastal and marine environments

Marine and coastal areas are classified in this paper according to attributes of the physical environment and by the faunal assemblages involved. Oceanic and coastal-margin realms are defined and mapped with the marginal seas and archipelagos named according to their connections to oceanic realms. There are seven types of ocean realms and thirteen types of coastal-margin realms. Latitudinal symmetry is present between hemispheres and from ocean basin to ocean basin. The defining characteristics for these realms are the seasonal variations in ocean surface currents and the companion seasonal variations in the main wind-currents of the atmosphere. Forty faunal provinces are indicated on the six maps of the oceanic and coastal realms. As sufficient database for their inclusion is not yet available, coastal marine and coastal terrestrial floristics and vegetation are not considered in this classification. While the boundaries between faunal provinces do not always match boundaries between ocean and coastal realms, the foundation is laid for a binominal designation system with the coastal or oceanic realm as ‘genus’ and the faunal province as the ‘epithet’—e.g. Western Subtropical—Cortezian. With this system, global intercomparisons become manageable. For example, the Western Subtropical-Cortezian can be directly contrasted with the Western Subtropical—Louisianian and the Western Subtropical—Southwestern Australian.

Coastal Marine Fauna and Marine Climates of the Americas

Geographic ranges of 968 marine species along coastal areas of the Americas, excluding the Arctic, were analysedfor co-ranges and compared with recent classifications of coastal marine and wave climates. Forty-two co-ranges and thirty-nine characteristic endpoints are reported. Many faunal boundaries correspond to those of previous studies, and thirty of the thirty-nine characteristic endpoints coincide with transition regions of the physical environment previously determined through analysis of motion fields of marine and atmospheric fluids. The hydrodynamics of the coastal marine environment give rise to the extant thermal structure of coastal waters and as such to the faunal distributional patterns exhibited. The high level of coincidence between biotic and abiotic zones of change along the coast clearly merits the use of both coastal marine fauna and the hydrodynamics of adjacent waters to establish the marine provinces of the coastal environment.

Effects of Hurricane Ginger on the Barrier Islands of North Carolina

The two barrier-island systems of North Carolina responded to the storm waves and surges of Hurricane Ginger in a strikingly different manner. Within the northern sector, which has been stabilized by man, erosion and dune recession were extensive. In the southern sector, as yet relatively unmodified, overwash and associated deposition were the dominant processes. This difference offers important geologic, ecologic, and land management implications.

Crescentic Landforms along the Atlantic Coast of the United States

An uninterrupted series of hierarchically arranged crescentic coastal landforms occurs along the Atlantic coast. Dimensional analysis suggests a geometric relation between order, amplitude, and number. The processes responsible for these features also may be continuous in nature.

Barrier Dune System along the Outer Banks of North Carolina: A Reappraisal

Barrier dune development has been encouraged by man along the Outer Banks of North Carolina to stabilize the barrier islands. This modification of a delicately balanced natural system is leading to severe adjustments in both geological and ecological processes.

Shoreline erosion rates along the middle Atlantic coast of the United States

A method has been developed and tested for mapping shoreline changes using an orthogonal grid system and historical aerial photography. Our data base now includes rates and variances of shoreline change and storm-surge penetration at 100-m intervals along the coast for 630 km between New Jersey and North Carolina. Analysis of the data shows that the average rate of shoreline recession is 1.5 m/yr; however, variance along the 630-km reach is high, with rates of as much as 10 m/yr for the Virginia barrier islands.

Storms and Shoreline Configuration

ABSTRACT Spectral analysis of the 1962 great Atlantic coast storm penetration (overwash) along the Outer Banks of North Carolina and Fenwick Island, Maryland, reveals along-the-coast periodicities ranging in wavelength from 14 km to 15 km. Periodicities with similar wavelengths exist in long-term mean rates of change of the shoreline and storm-surge penetration line. This suggests the location and magnitude of storm deposits and storm hazards along the Atlantic coast are systematically distributed.