Late glacial and Holocene history of the Penobscot River in the Penobscot Lowland, Maine

Abstract

When the Laurentide ice sheet retreated rapidly (~150 m/a) across the Penobscot Lowland between ~16 and ~15 ka, the area was isostatically depressed and became inundated by the sea. Silt and clay were deposited, but no significant moraines or deltas were formed. The Penobscot River was reborn at ~14 ka when ice retreated onto land in the upper reaches of the river’s East Branch. As isostatic rebound exceeded sea level rise from melting ice, the river extended itself southward. Between ~13.4 and 12.8 ka, it established a course across marine clay and underlying glacial till in the Lowland. Its gradient was low as differential rebound had not begun. Discharge, however, was higher and the river transported and deposited outwash gravel. During the cold, dry Younger Dryas, ~11 ka, eolian sand began to accumulate in dunes in the Lowland. Some of this sand, along with fluvial sediment from the headwaters, was redistributed into terraces along gentler stretches of the river and into a paleodelta in Penobscot Bay. Eolian activity continued to ~8 ka and aggradation in terraces until ~6 ka. The climate became wetter and warmer after ~6 ka, the dunes were stabilized by vegetation, the river began to downcut, and braiding became less intense. Pauses in the downcutting are reflected in discontinuous strath terraces. In due course, the river re-encountered the old outwash gravels, marine clay, glacial till, and, in a few places, bedrock. Its profile is now stepped, with gentle, gravel-bedded reaches between bedrock ribs that form rapids.

River channel response to dam removals on the lower Penobscot River, Maine, United States

“…The study area extends from the Milford Dam to the head‐of‐tide at Eddington Bend just below Veazie Dam (Figure 1a). In this reach, the channel gradient is 0.002, about five times steeper than the gradient of the river upstream for about 30 km above the Milford Dam impoundment (Hooke, Hanson, Belknap, & Kelley, 2017). This uncommon longitudinal profile—steepening nearest the coast—is the result of the interaction of the varied bedrock geology, glacial processes, and a complex sea level history.…”

“…Upstream of the Milford Dam, the area is underlain by largely uniform metasedimentary rocks. Downstream, channel incision exposed resistant quartz‐rich beds in the Paleozoic metamorphic rocks (Hooke et al, 2017; Kelley, 2006; Kelley, Kelley, Belknap, & Gontz, 2011). These beds trend approximately perpendicular to the flow direction and form a sequence of local base levels, creating falls and rapids that extend for approximately 22 rkm to the head‐of‐tide (Kelley, 2006).…”

“…Penobscot River sediment transport competence later diminished with lower meltwater input and a drainage divide shift at approximately 10 ka that decreased discharge by about 15%. This transition was marked by a shift in floodplain deposition from sand‐sized and coarser deposits to fine‐grained sedimentation (Hooke et al, 2017; Kelley et al, 2011).…”

River Reach Restored by Dam Removal Offers Suitable Spawning Habitat for Endangered Shortnose Sturgeon

“…Isostatic adjustments linked to glacial retreat and evacuation of outwash deposits also decreased the longitudinal gradient of the river, reducing sediment transport competence—specifically the ability of the flow to move gravels and larger‐sized sediment particles (Hooke et al. ).…”

“…Deposits of gravel‐sized or smaller sediment that do exist are mostly stored in the Island Division of the river, where bedrock outcrops have created gentle gradients (Kelley ; Hooke et al. ).…”