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Allegheny River islands remain protected

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By Paul G. Wiegman
Sunday, July 6, 2008
 

I am always surprised when I am reminded of something so obvious that it gets to be overlooked. That happened recently when I was asked by a reporter from a local radio station to talk about the islands in the Allegheny River.

I see one or more of the islands most every day when I'm in Pittsburgh. However, I never had given them much thought until asked. The reporter was interested in the way these pieces of dry land in the middle of the river formed, how they were used and what the future might hold.

Starting at the Point in Pittsburgh and going upstream, the islands include Herrs or Washington's Landing, 7-Mile just below the Highland Park Dam, Sycamore and 9-Mile near the mouth of Sandy Creek, 12-Mile just upstream of the Hulton Bridge and 14-Mile, where the Pennsylvania Turnpike crosses the river. The islands with mile designations reflect their distance from the Point.

There are even more islands farther upstream. Some are extensive archipelagos of large and small bits of dry land interlaced with braids of free flowing river. Siggias Islands, just south of Tidioute, is a maze of land and water -- a rich, dense landscape of trees, ferns and shallow water to be explored by foot and canoe.

By comparison, the Allegheny River is island-rich, while the Monongahela is island-poor. The reason for this disparity is largely because of the land drained by each river.

The Allegheny River is relatively new, geologically.

Before the last glaciation, the river was three separate streams. The great glacial ice sheets dammed those streams flowing on the pre-glaciation landscape of northwest Pennsylvania. When dammed, enormous lakes formed in the stream valleys and finally overflowed. Where they breeched the watershed divides, they merged and cut a new path. This new river roughly followed the front edge of the continental glacial sheet.

When the glacial sheets melted, the raw, gouged, exposed land was strewn with deep layers of glacial mud, sand, gravel and boulders. Geologists call this mixture till. The unsorted till was attacked by the elements, and the debris left by the ice was carried away by streams flowing into the new Allegheny River. The load of sediments was enormous, and much was deposited in the valleys and on the bed of the river. In some places, the sands and gravels deepened until, finally, they were piled above the non-flood surface of the river, thus forming the islands.

The Monongahela River drains an old landscape to the south, one that never was touched by glaciers. Thus, the sand and gravel from which islands are formed was sparse, and Monongahela River islands are few and far between.

Today, the Allegheny might be viewed as two very different rivers.

The lower, or most downstream, part has been altered with a chain of dams and locks. These maintain a regular river level that allows large boats, especially commercial barges, to navigate the river at all times of the year. Without the navigation system, river levels would fluctuate drastically because of spring floods that would pose a danger to river travel and low water in the summer, when anything larger than a canoe would be hopelessly grounded. Safe travel would be limited to just a few months or weeks each year.

The navigation dams and locks on the Allegheny River stretch from the Highland Park Bridge, Dam & Lock No. 1, to the Dam and Lock No. 9 at East Brady, 62 miles from the Point. There is deep water for a few more miles to the end of commercial navigation at the Route 68 East Brady Bridge over the Allegheny River. From that point upstream, the Allegheny is a free-flowing river.

On the wild-river section, the waterway alternates between deep, slow-moving pools and fast-flowing riffles with shallow, gravel-lined bottoms. The riffles usually are associated with islands that are just a few feet above river level.

There is one large dam, just upstream of Warren, at Kinzua Reservoir. The huge structure holds back the Allegheny River in a pool that reaches 24 miles and into New York. This lake is designed for flood control, not navigation. Upstream of Kinzua is more free-flowing Allegheny River.

River islands aren't permanent features. They are temporary land that is forever changing in size and shape, and even sluggishly drifting downstream.

At the upstream end of an island, water chews away at the land, carrying away the accumulated pile of soil, sand and gravel. If the island has sufficient vegetation, especially trees with broad, tangled root systems, erosion is slowed but never stopped.

At the downstream end of an island, the river slows and eddies. Without the energy of the faster water to hold suspended sands and gravels eroded from the head, they fall back out of the water and are deposited in the channel. A long, pointed tail of sediment first forms shallows, then dry land, lengthening the island on the downstream end.

The combination of erosion at the head and sedimentation at the tail give the appearance that the island is slowing moving downstream. This changing island landscape is more apparent in the free-flowing parts of the river. Dams and locks slow river water, and the islands are less affected by erosion and sedimentation.

Because river islands are built from sand and gravel, they have been considered a valuable natural resource to be mined. Some islands were completely removed by dredging and others significantly reduced in size. When it was realized that these environments are valuable as shallow waters for fish breeding, have rare fresh water mussels in the riffles, are excellent feeding and resting grounds for migrating waterfowl and other birds and are a unique habitat unto themselves, regulations were instituted to protect them.

The ecological value of islands for birds didn't escape the Audubon Society of Western Pennsylvania. In 1936, the Audubon Society leased Sycamore and 9-Mile islands from the Pennsylvania Railroad. These became the first bird sanctuaries in the region. Bird watchers regularly visited and patrolled the islands via canoes to enjoy the rich waves of spring and fall migrating species. Later, the land was sold to Davidson Sand & Gravel with the anticipation of dredging; the Audubon lease was canceled.

Davidson also bought 12-Mile and 14-Mile islands. Dredging never took place, and the company passed those islands on to the Western Pennsylvania Conservancy during the 1970s. Later, the conservancy transferred ownership of most of 12-Mile Island to people with cabins or homes on that island. Restrictions were included in the transfer to limit development and protect against actions that might cause excessive erosion.

14-Mile Island, divided into two large parts by Dam and Lock No. 3 at Harmar, never had been extensively developed. A pier for the Pennsylvania Turnpike was placed on the island, but otherwise it remains a wild island covered with sycamore, silver maples and willow. In 1980, the conservancy transferred 14-Mile Island to the Pennsylvania Department of Conservation and Natural Resources. It is now Allegheny River Islands State Park, a 43-acre wild island oasis within easy access of Pittsburgh.

Sycamore Island also is protected land. In December 2007, the equally wild island was bought by the Allegheny Land Trust. This was one of the islands recognized by the Audubon Society back in 1936 as an important sanctuary, and after 71 years, the vision of full protection of the land has been accomplished. Sycamore Island is hardwood floodplain forest of mostly silver maple.

We are fortunate to have important natural lands available for protection, and the groups that have taken the initiative to bring that protection to fruition deserve our gratitude.

 

 
 


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