Androscoggin Lake's Outlet Delta System

Introduction

Maine has over 5000 lakes and ponds that are at least one acre in size. Many of these water bodies, especially the larger ones, have one or more inlet streams that bring surface water into the lake, and an outlet stream that drains water away from the lake. Outlet streams commonly lead to a larger river (trunk stream) farther down the drainage basin, at a lower elevation. Normally an outlet stream always flows away from the lake that it's draining. In rare cases, however, there is very little difference in elevation between the lake, its outlet stream, and the trunk stream. When the major river floods, it may overwhelm the small stream and force it to reverse direction and flow back into the lake. This causes water to pour into the lake at the point which is usually its outlet! During the flood event, fine sandy or muddy sediment is carried into the lake and builds up to form a flat-topped deposit called a "lake-outlet delta."

diagram of normal and high-water flow at a meander

Figure 1

In most situations where a lake-outlet delta has formed, the outlet stream of the lake joins the trunk river at an acute angle and along the outside of a meander bend (Caldwell, 1989). During flood periods, the water surface tends to "pile up" on the outside of the meander. This situation favors the movement of water into the outlet and contributes to the reversal in flow (Figure 1). There are also small islands at the mouths of many outlet streams. These likely formed as a result of elevated flow from the lake when flooding of the trunk river subsides and the normal drainage direction is restored.

The Outlet Delta on Androscoggin Lake

There are 14 lake-outlets in Maine (Figure 2), and one of the most well developed of these is on Androscoggin Lake in the towns of Leeds and Wayne (Figure 2 and Figure 3). Androscoggin Lake drains into the sluggish Dead River, which in turn flows north for several miles and empties into the Androscoggin River. At the close of the Ice Age, the Dead River would have had a steeper slope to the north, but postglacial uplift and gentle southward tilting of the earth's crust have reduced the gradient of this stream. Since there is now only a very slight drop in elevation between the lake and the Androscoggin River, flooding of the Androscoggin forces the Dead River to reverse direction and flow back into the lake.

location map of Maine's lake outlet deltas

Figure 2

regional topographic map of Androscoggin Lake outlet delta

Figure 3

Lakes containing outlet deltas often lie within the flood plain of the nearby trunk river. However, the Androscoggin Lake outlet delta is situated nearly 7 miles from the Androscoggin River. In terms of distance from the trunk stream, the Androscoggin Lake outlet delta is second only to the Pushaw Lake outlet delta which is located approximately 7.5 miles from the Penobscot River. The Androscoggin Lake outlet delta is very large, extending a distance of nearly two miles out into the lake (Figure 4 and Figure 5).

Detailed topographic map of the Androscoggin Lake outlet delta

Figure 4

Figure 5

The shapes of Maine's lake-outlet deltas are influenced by the combination of fluvial action by the contributing streams and wave action on the lake. Where wave action predominates, the delta usually runs along the lakeshore, with the stream separated from the lake by a narrow spit. In contrast, where river action predominates, the delta is allowed to grow out into the lake for a considerable distance. The Androscoggin Lake delta belongs to the latter type and is analogous in this respect to the Mississippi River's "bird-foot delta" (Figure 6a-e). It is believed that the length of the Androscoggin Lake delta has resulted from the frequency of flood events in combination with the shallow nearshore conditions (Androscoggin Lake has a maximum depth of 38 feet, but is typically less than 20 feet deep). Also, the till-covered bedrock islands at the delta terminus protect it against erosion by waves. In the last 40 years, the delta has expanded to connect with most of the islands shown on the 1966 topographic map of the Wayne quadrangle (Figure 7).

Reference

Caldwell, D. W., FitzGerald, D. M., and Fenster, M. S., 1989, Origin and sedimentation of Maine lakes with emphasis on lake-outlet deltas, in Tucker, R. D., and Marvinney, R. G. (editors), Studies in Maine geology; Volume 5 - Quaternary geology: Maine Geological survey, p. 97-108.

Originally published on the web as the April 2005 Site of the Month.

Last updated on October 6, 2005