Editor’s Note: This is a column on area watersheds by Blyden Potts and guest columnists to spread awareness of the area’s tributaries and the efforts of area volunteers to keep them clean.
Like people, streams are shaped by their surroundings and change over their life course, as they flow from headwaters to mouth. The changes they undergo are shaped by physical features such as slope and type of bedrock, and other geologic structures like folds, fractures, and fault lines. Conodoguinet Creek, known affectionately as “Connie,” is a good model of these effects.
In her youth, Connie runs fast and relatively straight for about 15 miles, constrained to flow northeasterly.
The narrow valley between Little and Timmons Mountains is what geologists call a syncline: a canoe-shaped geologic fold with hard, erosion-resistant sandstone rock layers forming the ridges, the gunwales of the canoe, and soft shale rocks in the valley bottom. The stream starts above 1,600 feet elevation and erodes down about 800 feet into relatively soft bedrock, dropping about 50 feet every mile. The energy of the descent has significant erosion power, resulting in a bedrock streambed frequently overlaid with a gravel mantle.
After passing through Letterkenny Reservoir, Connie changes course, breaking the hull of the canoe as she drops through the gap at Roxbury. The gap is wider, but the stream here can be as narrow as the length of a canoe.
From there, Connie flows southeast for about 8 miles. Snaking and braiding through more soft rock, she drops over small ledges of slightly more resistant rock. In this segment, she erodes downward another 250 feet at a stream gradient of about 30 feet per mile. Fractures within the bedrock, that run northwest to southeast, are the likely reason she flows in this direction. They weaken the rock, providing an easier path for stream flow.
Near Orrstown, Connie once again turns northeasterly, slowing and widening. The stream flows roughly parallel to the ridge, along or near the line of contact between more and less resistant rock layers. Yet four or five times it elbows back to the northwest, turns sharply back to the southeast, and resumes its northeast flow toward the Susquehanna River. This repeated pattern again suggests she is following pathways where fractures have weakened the bedrock. Here the kayaker begins to see shoals and higher banks of sediments deposited by the stream, resulting from the lower stream velocity.
North of the Conodoguinet, hundreds of tiny, ephemeral streams run down the slope of Blue Mountain and erode into shales. South of the stream, limestone karst allows water to sink underground into the porous “swiss cheese” bedrock, re-emerging from springs along the valley. This is why there are fewer tributaries on the south bank. The main exception is our own Middle Spring Watershed, with surface water coming off the Michaux.
Near Green Spring, about 20 river miles from Roxbury Gap, Connie begins a new pattern, making wide meanders, with major oxbows entrenched in the shale. The first oxbow occurs at State Game Lands No. 169. The second at Grahams Woods. As she approaches Carlisle, the meandering becomes more pronounced and consistent. As she reaches Mechanicsburg, the oxbows become elongated, very tight, and deeply incised relative to surrounding land. These meanders are the source of the name Conodoguinet, said to mean “many bends.”
At many points, Connie passes over and through stone ridges she has eroded into. Water and sediments tend to pool upstream of each natural dam ridge. This reach of the stream erodes downward less than 150 feet over a distance of about thirty miles: a very gentle stream gradient of about 5 feet per mile.
Connie is a different creek in different sections of her course, shaped everywhere by underlying geology.