DELAWARE RIVER BASIN COMMISSION

Overview

The following narrative illustrates the natural functions and problems associated with waterway corridors.

Natural Functions

If properly managed, streams and the surrounding corridors can bring a variety of environmental, economic, and cultural benefits to a community. Protecting the habitat of diverse species, protecting and enhancing water quality and supply, and providing recreation are major considerations.

A stream in a relatively undisturbed condition possesses resilient channel stability. Its properties include relatively stable dimensions in plan, profile, and cross-section. (The degree of the inherent, natural stability characteristics of the basin streams can only be determined through implementation of a monitoring program.) A stream transports its sediment load without changing its channel elevation, and without excessive siltation of the stream bottom. Certain disturbances upset channel stability and the resultant channel adjustments may deteriorate environmental, economic, and cultural attributes. Such a stream is described as unstable.

Leopold (1994) summarized the physical laws impacting stream geomorphology. Stream channel width, depth, slope, meander pattern, and flood plain contours; bed load transport rates; and other geomorphologic features develop through interaction of flowing water with valley soil and plant material. After a stream segment is severely disturbed, the resultant evolution to a reconfigured stable form involves larger-scale instability and the long-term negative effects of increased erosion and sedimentation.

Rosgen (1996) developed the Rosgen Stream Classification System to predict stream geomorphology behavior based on readily-measured physical features. The system, among its other uses, allows translation of dimensionless ratio data from a relatively undisturbed segment of a stream to a similar (by geology, topography, and climate), yet altered stream segment. This translation of dimensionless ratio data, such as width/depth ratio, when applied judiciously, is particularly valuable as a design template for restoration projects that involve substantial reshaping of channels and flood plains of severely deteriorated stream segments.

A stream corridor in a relatively undisturbed setting is normally well vegetated, and meanders as a green ribbon from headwaters to its mouth. Well-vegetated corridors serve as a line of defense to protect water quality and aquatic habitat. Such corridors mitigate the upland assaults of nonpoint source pollution related to development, and improper agricultural and forest harvest activities in the watershed. The riparian vegetation also stabilizes streambank soils against erosion. The shielding canopy of riparian vegetation maintains cold water fisheries.

A well-planned "green way" corridor that supports a community walkway, a park, wildlife habitat, or a creatively planned housing development or commercial activity helps distinguish pleasant communities from those that are blighted.

A significant report on defining wetlands (National Research Council, 1995) provides recommendations for the protection of stream corridors:

Riparian zones perform many of the same functions as do wetlands, riparian zones cannot be defined wholly as wetlands...If national policy extends to protection of riparian zones pursuant to the goals of the Clean Water Act, regulation must be achieved through legislation that recognizes the special attributes of these landscape features, and not by attempting to define them as wetlands.

Problems

The Delaware River Basin and its myriad tributaries have substantial expanses of green way corridors. However, all is not well. Stream bank erosion and resultant siltation of stream channels appears to be accelerating. Clearing of riparian vegetation and increasing flood flows appear to be dominant causes.

Clearing of vegetation is a result of diverse activities. Private and public lawns are mowed to the top of stream banks. Regrading for land development projects removes some riparian vegetation. Livestock have access to many miles of streams. Agricultural cropping is quite often close to streams.

The transition from pervious to impervious surfaces as land develops increases the volume and peak flow associated with stormwater runoff. Increased stormwater runoff increases peak stream flows, which tends to increase the width between stream banks. Hence during dry-weather flows, depths are more shallow and more exposed to direct sunlight. These effects decrease habitat and recreational value.

Loss of channel stability may cause either aggradation or degradation of the channel. Aggradation is the raising of a stream bed elevation, which increases flood levels and induces periodic, partial clogging of culverts and bridges. Degradation is the downcutting of a stream bed elevation, possibly to the extent that a stream loses contact with its former flood plain. The resultant loss of valley storage causes higher flood flows downstream. These and other impacts translate into added costs to communities through increased flooding, increased cost to maintain the transportation infrastructure of culverts and bridge spans, increased cost of treating water supplies, and decreased recreational opportunities through degradation or loss of fisheries habitat.

Road construction has some deleterious impacts on WWCs. Few of those constructed in the Delaware River Basin have controls to limit peak stormwater runoff. For roads that parallel streams, the combined effects of the removal of shading canopy and stormwater runoff from hot asphalt pavement may thermally stress fish. Additionally, cool-water seepages, important as fish habitat during the summer, may be lost through construction. Ideally a road's stream crossing should have a channel width/depth and flood plain configuration similar to a stream's natural condition Rosgen and Hey (1998). This would allow maintaining natural shear stress and resultant natural transport of the stream's bed load. As shear stress is proportional to water depth and slope, maintaining an overly wide or narrow opening in a bridge crossing would alter channel depth and resultant shear stress and could trigger channel destabilization. Destabilization might involve clogging of stream crossing structures and the resultant need for costly and environmentally destructive maintenance clearance.

Improperly sited and designed urban and suburban development also can severely impact the green way features and aesthetics of a waterway corridor. In the Delaware River Basin there is no shortage of examples of sprawl where, for example, commercial strip development is backed up too close to streams to support a natural stream corridor and is a blight to the landscape. A typical urban waterway corridor in the basin may be relegated to little more than an ugly, derelict stream channel providing nothing more than a repository for shopping carts, worn tires, scrap metal, and any other kind of trash imaginable.

The problems are diverse and many, as are the potential solutions. The Basin community needs to work in concert to maintain and restore its waterway corridors.