The History of Impacts of Sand and Gravel Dredging on the Allegheny River

Before commercial navigation brought alterations to the river, the lower Allegheny flowed over ripples and into natural pools. The ripples were formed by shallow deposits of sand, and gravel.

Sand and Gravel Imported by Glacial Activity
From as high as 300 feet on the high river terraces above the Allegheny River, to as low as 130 feet (in 1909) below water level, pebbles of granite, diorite, and other igneous rocks from Canada, and deep-red Medina sandstone from New York, are found among sand, silt, and clay.[1] These pebbles, or gravel, were once large rocks and boulders plucked by glaciers traveling south from Canada. Three Ice Age glaciers reached into Pennsylvania at Oil City and a little below Franklin, and crossed the Allegheny below Olean, NewYork.[2] Each glacier, potentially hundreds of feet thick, transported the foreign rocks and boulders, grinding them down through sheer weight and distance into the rounded pebbles that we know as gravel. Much of the gravel was further worn down into sand. Unlike the sand found in the Monongahela, which is derived from sedimentary rocks, the sand of the Allegheny and Ohio valleys comes from the igneous rocks to the north and is, therefore, hard, sharp, and clean. These qualities make it an historically valued resource by the building industry.

Collecting Sand and Gravel
During the first three or four decades of the nineteenth century, in seasons of low water, people would go into the riverbed, especially the Allegheny, to collect the large round cobbles to pave their streets and wharves. By 1850, the stones had become scarce enough that transporting them locally from upriver became a viable business. During low water, large flatboats were pulled upriver by horses walking along the river bank, cobbles and gravel were collected, and the boats were maneuvered back to towns. The first boat fitted with a clawing device for digging up materials from the river bed was the Higbee; it plied the Allegheny River in the late 1870s. Its first trip was made to help meet the demand for paving materials for residents along the Ohio River. In 1879, a towboat, called the Kangaroo, arrived at Cincinnati, Ohio, with five barges of Allegheny River cobbles and boulders to be used for paving.

To supply the area's growing glass-making industry, sand was collected from bars and shoals along Pittsburgh riverbanks. In low water, teams of horses drew wagons out onto the rivers (at low flow), or small flats were pushed about and sand was shoveled into them. As the business grew, larger flatboats were used. By 1852, the first steam-powered sand digger raised sand from the bottom of the river and placed it into flatboats. In 1854, J.K. Davison & Brother, today known as Davison Sand & Gravel Company, was established. With the introduction of concrete, in the 1870s, the demand for both sand and gravel went up, and commercial extraction became a viable business.

The second half of the 19th century saw the formation of more sand and gravel companies. In 1903, the Pittsburgh Gazette reported that every year over 1.5 million tons of sand were taken from the Monongahela and Allegheny Rivers by five sand and gravel companies in Pittsburgh. Sand from the Allegheny River was considered the best because it is hard and has a high silica content, desirable for both building purposes and glass-making. As early as 1909, Allegheny County alone produced approximately 2.3 million tons of sand and gravel, most of the digging occured in the lower Allegheny River and on the upper Ohio River.[3]

Modern Dredging Process
Dredgers on the Allegheny River today typically use the clamshell dredge, a heavy, hinged bucket shaped like the two halves of a clam shell. The bucket lowered into the river substrate, the two halves close, capturing sand and gravel, and lift the materials up and onto a barge. The process removes macroinvertebrates along with sand and gravel, produces turbulence, and leaves behind a pockmarked surface.[4] If the dredge has an on-board processor, the desired material is sorted from the rest of the dredged material and the fines, or silts, are discharged back into the river through a submerged diffuser. The diffuser releases fines into the bottom of the dredged area to reduce the amount of suspended solids that would travel downriver.[5]

The River Habitat and Impacts of Dredging
From Pittsburgh to East Brady, the lower Allegheny is dredged for both sand and gravel and to maintain a nine-foot navigation channel for the barge industry. Eight locks and dams built for navigation have changed the flow of the Allegheny from what was once a series of shallow riffles and pools to a series submerged riffle areas and deeper pools.[6] For 60 years, the flow of the lower Allegheny more resembles a succession of shallow lakes than a free-flowing river. Though good for commercial navigation and recreational boating, the altered river mechanics dramatically changed underwater and wetland habitats that long supported a diversity of fish, birds, insects, amphibians, and many freshwater species. Navigation charts show that riverbed scour (the natural transport of sand and gravel) has virtually stopped, except during major floods. If maintenance dredging were to stop, the current lake-like, or lentic, biota would gradually be replaced by a running water, or lotic, biota.

Impacts Dredging alters the substrate that supports underwater plant and animal species. It removes the coarser material that benthic (bottom-dwelling) organisms use for habitat. Because gravel is fairly stable and provides shelter and food for benthic organisms and benthic feeders, it is considered much more productive in supporting underwater life than silt or sand.[7] Much of the total fines returned to the river consist of very fine sand. One of the most adverse consequences of fine sediments is that they settle on the bottom of the dredging pits or further downstream where they accumulate with organic material and decompose without oxygen. These areas then become anoxic, which means they generate no oxygen to support aquatic life. In addition, the process of discharging sediments creates turbidity that suspends the fine particles in water and disperses them downstream, not unlike smoke from a smokestack, traveling downwind. Increased turbidity reduces light penetration and can decrease the critical process of photosynthesis practiced by phytoplankton, attached algae, and aquatic vegetation—the building blocks of a wide river food chain. Organisms higher up the food chain can be affected if high turbidity levels persist.[8] Fine sediments also settle on fish eggs and cause a high mortality rate. Macroinvertebrates may be smothered, thereby reducing, sometimes eliminating an important food source for fish. Lastly, increased turbidity impairs the vision and respiration of fish.

Dredged v. Not Dredged Benthic Habitat
Pool 6 of the Allegheny River contains a large portion of natural river bottom that has never been dredged and is relatively shallow, providing quality habitat for numerous fish species and other aquatic life. Recognizing this fragile habitat, the Pennsylvania Department of Environmental Resources placed a ten-year moratorium on dredging in Pool 6. The moratorium went into effect in December 1985 and was still in effect in early 1999. During the moratorium, resource agencies were charged with assessing the impacts of dredging.

Pennsylvania Fish and Boat Commission Study
The Pennsylvania Fish and Boat Commission performed a study to assess the impacts of dredging and chose to compare the more natural Pool 6 with the excessively dredged and deep Pool 5, which is nearly devoid of natural river bottom.[9] The purpose of the assessment was to determine the diversity of fish species in each pool, and to establish relative numbers of game and panfish present. The study found that while both Pools 5 and 6 contain diverse fish populations, those in Pool 6 appear to have attained their diversity through reproductive success and completion of life cycles within the pool. Pool 5, by contrast, had smaller populations and no individuals of certain forage species. This indicates that the Pool 5 populations had little to no reproductive success, and that fish populations from adjacent pools and tributaries are not migrating into Pool 5. The study's conclusion: overall fish productivity is considerably higher in Pool 6 than Pool 5.

Species absent from Pool 5 but present in Pool 6 are mostly all dependent on one or more of the following habitat requirements:
1) Absence of silt or sediment build up;
2) Clean sand and gravel substrate;
3) Shallow flowing waters/riffles

Dredging removes all three of these habitat traits. The fact that all three still exist in Pool 6 explains the greater diversity of fish populations there (including dense smallmouth bass and walleye).

U.S. Army Corps of Engineers Study
The U.S. Army Corps of Engineers Pittsburgh District, also performed a study comparing Pools 5 and 6. Their study focused on three factors: substrate size ; benthic macroinvertebrate communities; and background water quality.

Substrate particle sizes were determined using four categories for gravel (the largest being cobble) and three for sand and fine sediment (silts and clays). The Corps found the substrate of both pools to be generally coarse. However, there was about 19 percent more cobble in Pool 6 than in Pool 5. And over 10 percent more silts and clays in the substrate of Pool 5.

The amount of gravels, sand, and silt affects the distribution of benthic macroinvertebrates living in these pools. There were twice twice as many living in Pool 6 compared to Pool 5. Also, the Pool 6 samples had significantly less Corbicula (Asian clams). The likely reason: shallower waters generally have more diverse and more productive invertebrate communities.

The water quality tests show little difference between the water of the two pools. This was expected due to the watersheds feeding into them, which are characterized by relatively little tributary drainage. Of the 9,351 square miles draining into the Allegheny at Lock and Dam 5, only 19 square miles, or 0.2 percent of the drainage is local, as a result, 99.8 percent of the water in Pool 5 is received from Pool 6. The total drainage area at Lock and Dam 6 is 9,332 square miles, of which only 350 miles or 3.8 percent of the total is from local drainage tributary. Acid mine drainage does emanate from the 369 square miles of local drainage into pools 5 and 6. The impact of acid mine drainage pollution, however, was not even measurable at the mid-channel and/or the lock and dams' monitoring stations.

Lead concentrations, however, were higher in Pool 5 (6.9 ug/l) than Pool 6 (2.3 ug/l). And differences in the amount of dissolved oxygen sampled in both pools was negligible.[10]

Wetland Study
In a special study conducted by the Fish and Game Commission, Fish and Wildlife biologist, Richard McCoy, surveyed the wetlands in Pools 5 and 6. Wetlands provide food, cover, and nesting habitat for a variety of shore birds, waterfowl, mammals, reptiles, and amphibians. Numerous black duck, mallards, and great-blue herons were observed around the Cogley Island complex in Pool 6. Some of these wetlands also provide valuable spawning, nursery, and feeding areas for many river fishes. Rare fishes still inhabit the shallow-water wetlands around Cogley Island including bluebreast darter, channel darter, goldeye, longhead darter, and river redhorse.

Approximately 118 acres of wetlands (42 acres emergent, 7 acres scrub-shrub, 68 acres forested, and 1 acre submerged aquatic bed) exist in Pool 6. With 71 species of plants identified, the wetlands of Pool 6 were much more diverse than those of Pool 5. Emergent wetlands varied from almost solid stands of reed-canary grass, water- willow, and smartweed, to the highly diverse wetlands in the back channels of Cogley Island. Dominant plants included spotted jewelweed, wingstem, tall coneflower, smartweeds, water-willow, goldenrods, and rice cut-grass. Cogley Island is the largest riverine wetland found along the Allegheny River.

In contrast, there are less than 12 acres of wetland in Pool 5, with almost 3 acres of emergent/scrub-shrub and 9.0 acres of forested wetlands. These wetlands were restricted to river mile 32.8 near Donley Island. The forested wetlands were dominated by silver maple with spicebush in the understory, and spotted jewelweed and wingstem co-dominating the herbaceous layer. False nettle and white snakeroot were also common. The emergent/scrub-shrub wetlands were created by beaver activity and an abandoned railroad bed.

The differences in substrate and other physical characteristics between the two pools and surrounding landscape account for the difference in wetland acreage.[11]

Click here to see a sonar image of clamshell dredging pits on the bottom of the Allegheny River.

[1] Mrs. S. Kussart, The Allegheny River, (Pittsburgh: Mrs. S. Kussart, 1938), p.281.

[2] John A. Harper, "Of Ice and Waters Flowing: The Formation of Pittsburgh's Three Rivers," Pennsylvania Geology , Vol. 28, No. 3/4, p. 3.

[3] Kussart, p.295.

[4] U.S. Army Engineer District, Pittsburgh, Corps of Engineers, Final Environmental Statement on Allegheny River Pennsylvania (Mile 0 to Mile 62.2), Commercial Sand and Dredging Operations, (Pittsburgh: U.S. Army Engineer District, Pittsburgh, Corps of Engineers, 1980) p. 58.

[5] Interview with Bob Neil, Army Corps of Engineers, Pittsburgh District, 7/15/98.

[6] U.S. Army Engineer District, Pittsburgh, Corps of Engineers, p.45.

[7] Ronald Lee, Richard Lorson, Thomas Shervinskie, and Allen Woomer, Dredging Impacts Study, Allegheny River Pools 5 (Section 18) & 6 (Section 17), Armstrong County, Pennsylvania, (Pennsylvania Fish & Boat Commission, February 1992), p. 6.

[8] 21 U.S. Army Engineer District, Pittsburgh, Corps of Engineers, p.64.

[9] Lee, Lorson, Shervinskie, and Woomer, Executive Summary.

[10] U.S. Army Engineer District, Pittsburgh, Corps of Engineers, Commercial Sand and Gravel Dredging Impacts on Substrate Particle Size Distribution, Macroinvertebrate Communities, and the Water Quality of Allegheny River Pools 5 and 6, River Miles 30.4 to 45.7, (Pittsburgh: U.S. Army Engineer District, Pittsburgh, Corps of Engineers, 1993), pp. 11-13.

[11] Richard McCoy, Special Report 92-4, Lower Allegheny River Wetland Survey, River Miles 41.5-45.5, Navigation Pools 5 and 6, (Fish and Wildlife Service, 1992) pp.all.

This document is part of the Resource Center of the Watershed Atlas of the Allegheny River.