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Fall 2009

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Fall 2009 Edition

Photo by Steve Grund
Youghiogheny River Scour: An unusual place for unlikely plants with scene of Youghiogheny river

by Steve Grund and Ephraim Zimmerman-
Pennsylvania Natural Heritage Program

While kayaking or canoeing the rivers of Western Pennsylvania, you might have noticed patches of prairie-like grasslands found along the sand, cobble, boulder and bedrock banks. These ecosystems are home to many species of conservation concern, and they have become increasingly rare in Pennsylvania due to habitat conversion and damming.

They have also suffered from the onslaught of invasive alien plants. These prairie-like ecosystems are often found at the mouths of tributary streams, where small deltas called "cobble fans" form, as well as on islands, at rapids and where large boulders or exposed bedrock is directly in contact with the flowing water. They are definitely places you don't want to be during the spring floods! Known as ice-scour grasslands, riverbank bedrock communities, or river floodplain prairies, these grass and herb dominated ecosystems are pummeled each year by intense floodwaters and flowing sheets of ice that crash and scour the floodplains, at times down to the bedrock.

Flooding is a natural disturbance process on all streams and is important for maintenance of floodplain ecosystems of various kinds. On larger rivers, especially those that carry significant amounts of ice during spring melts, this process is very vigorous, ripping up much of the vegetation, creating a zone we call "river scour."

In Pennsylvania, significant river scour occurs on the Allegheny, Clarion, Conemaugh, Delaware, Susquehanna, Juniata and Youghiogheny rivers, as well as on some stretches of their major tributaries. On many rivers, especially the Monongahela, the lower Susquehanna and the Ohio, damming of the rivers for flood control, water supply and to form navigation pools and recreational lakes has highly modified or eliminated floodplain habitats, including scour.

The Youghiogheny River, specifically the stretch called the Youghiogheny Gorge, between the Youghiogheny Reservoir and Connellsville, is the most dynamic large river in Pennsylvania because of the steep descent it makes while cutting through Laurel Ridge and Chestnut Ridge. This steep gradient is why the rapids are there, and scour has kept much of the area along the stream free of substantial accumulations of soil.

From its origin in the mountains of Maryland and West Virginia, the Youghiogheny weaves its way through a variety of landscapes, rock formations and elevational gradients, resulting in a great diversity of scour habitats. Add to the habitat diversity the large number of Appalachian species that extend to the northern limits of their ranges in this deep river valley, which runs from south to north, and you get one of Pennsylvania's most important regions for biodiversity conservation.

Scour zones share ecological characteristics with river banks, floodplains, wetlands and prairies. This combination of conditions leads to a unique grouping of species generally associated with one or more of those ecosystems.

Often the most conspicuous component of scour habitats are what we tend to think of as prairie grasses, such as big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), switch grass (Panicum virgatum), cordgrass (Spartina pectinata) and Indian grass (Sorghastrum nutans). While commonly associated with the Midwestern prairies of Ohio, Michigan, Illinois and Wisconsin, these plants find a home in Pennsylvania among the scoured cobbles and bedrock of the river floodplains.

Photo by Steve Grund
Pink flowered swamp milkweed growing along the Youghiogheny River
Pink-flowered swamp milkweed grows in scoured cobble areas along river.

Despite the importance of these species, only a few other species usually associated with prairies are common in the scour areas along the Youghiogheny. These include wild-indigo (Baptisia tinctoria), tall tickseed (Coreopsis tripteris), flowering spurge (Euphorbia corollata), yellow star-grass (Hypoxis hirsuta), and early goldenrod (Solidago juncea).

A smaller number of the plants found in scour areas in the Youghiogheny River Gorge are species commonly found on stream banks and floodplains in less dynamic habitats. Perhaps the most conspicuous of these is a tree, the sycamore (Platanus occidentalis). Sycamores are common -- even locally abundant -- but rarely reach what we think of as tree size in this habitat. Some of the sycamores are gnarly, small but perhaps very old, having been repeatedly battered by the powerful forces of floods and ice scour. A few silver maple (Acer saccharinum) and green ash (Fraxinus pennsylvanica) are also present.

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