Migration of the Morse River into Back Dunes at Popham Beach State Park, Phippsburg, Maine

Introduction

Erosion at Popham Beach State Park continued to cut into the back dune pitch pine forest throughout 2009. The inland extent of erosion is farther than any recorded by geological measurements in the last 50 years and probably is the farthest inland the beach has moved in the last 100 years. Here we review erosion trends over the last 20 years and discover a pattern of dune loss that can be attributed to tidal currents flowing into and out of the Morse River with episodes of severe erosion during times of large storms and high tides.

Figure 1

Popham Beach is part of a large and complex beach system at the mouth of the Kennebec River (Figure 1). The dynamic nature of the beach and dune system has been studied by geologists from the University of Maine and Boston University as well as the Maine Geological Survey. We have previously reported on many aspects of this dynamic system including how the sand bar from the park to the Fox Islands was cut by the Morse River (Dickson, 2008a), how the spit at Seawall Beach has extended east and pinned the Morse River up against the state park (Dickson, 2008b), and how the Morse River began to show signs of changing its course to the sea in 2009 (Dickson, 2009).

Historical Shoreline Change Patterns

Shoreline change can be mapped from historical aerial photographs and combined with ground surveys using a global positioning system (GPS) to better analyze trends in erosion and accretion (dune growth). In the examples here, the shoreline is delineated by the seaward edge of dune vegetation (usually American beach grass) or the toe of a vertical scarp (embankment) just below the seaward edge of vegetation. By comparing shorelines over time we can see which areas have lost or gained dunes.

The first interval of time for comparison is from 1980 to 1986 (Figure 2). Over 6 years the dunes eroded significantly on the western portion of Popham Beach. This loss is adjacent to the Morse River and, with an understanding the movement and migration of rivers, we can be confident that the loss is due, in part, to an easterly shift in the river's course toward the park's dunes. The outer bend in the river (during its flow to the sea on a falling or ebb tide) is called a "cut bank" and it is this portion of the channel that has eroded the dunes. As we will see in the following few figures, the cut bank shifts position over time - demonstrating a geological process called meandering.

Figure 2

Figure 3

The second interval of comparison is from 1986 to 1991 (Figure 3). In this 5-year period some of the dunes lost previously grew back on the bank of the Morse River as well as the central dune field next to the large bar complex (tombolo; Dickson, 2008). This was an interval of net accretion to the dunes. The east portion of the state park beach remained relatively stable. Farther east along Hunnewell Beach the dunes also grew seaward.

From 1991 to 2003 the Morse River eroded some of the dunes created after 1986 (Figure 4). The rest of the dunes remained remarkably stable for this 12-year period. West of the Morse River the dunes on the east end of Seawall Beach expanded slightly.

Figure 4

Figure 5

Between 2003 and 2007 the Morse River removed a large area of dunes along the west beach (Figure 5). Initially, park dunes in the center beach lost area from 2003 to 2005 (the photographic base used in Figure 5 was taken in 2005). Erosion worsened after 2005. Severe erosion removed dunes for two more years and brought the shoreline up to the edge of the pitch pine maritime forest. The Morse River reoccupied a location not seen since perhaps the 1950s (see Figure 12 of Dickson, 2008a). While the dunes of the west and central beach areas were lost, the east beach shoreline was relatively stable. Hunnewell Beach also lost dunes in this 4-year interval.

From 2007 to 2009 the Morse River continued to remove both dunes and topple trees in the pitch pine forest (Figure 6). Erosion came closer and closer to the parking lot and new bath house facilities built in the spring and summer of 2009 at the southwest corner of the parking lot. The Morse River continued to meander closer and closer to park infrastructure.

Figure 6

Figure 7

Figure 7 summarizes shoreline changes since 1980 and shown individually in Figures 2 through 6. Since 1980 when the dunes were at their largest extent, Popham Beach State Park has lost a lot of sand dunes and pitch pine forest. The pattern of dune loss is one of cutting by the Morse River from west to east over nearly three decades. During that time, however, there was a period from about late 1980s through the 1990s when the Morse River did not erode the park's dunes in any large way. Early in the new millennium the Morse River became more active in its easterly meandering and more damaging to the state park dunes. The most dramatic loss of park land happened from 2005 to 2007, a relatively short period of time. This summary figure shows how very significant land loss can take place in only a few years as dunes are eroded by swift currents at a tidal inlet associated with a small river system.

Storm Erosion in December 2009

Storms contribute to abrupt shoreline change and loss of coastal sand dunes. The map series above records both the meandering of the Morse River as well as storm erosion. In late 2009, a series of fall and winter storms allowed the Morse River to become elevated and sweep away large amounts of sand in a matter of hours. A storm on December 12 elevated the tide an additional 1.5 feet and resulted in minor coastal flooding over the 12-foot level (Figure 8). Offshore waves in this storm were 12 feet high and they broke offshore of Popham Beach State Park and moved ashore as smaller waves that eroded the dunes (Figure 9 and Figure 10). Along low dunes, waves overtopped them and flooding extended inland (Figure 11).

In consultation with resource and regulatory agencies, the Department of Conservation decided to take action in December 2009 to slow the effects of storms with the goal of keeping the shoreline from reaching the bath house. Fallen pine trees were gathered from along the beach (Figure 12) and bundled with rope on the bank of the Morse River adjacent to the bath house to create a raft along the dune scarp (Figure 13). Ropes were used to tie the bundles to upright trees farther inland to hold them in place and to keep them from floating out to sea in storms (Figure 14). The goal of the "tree wall" was to slow the speed of the Morse River on the cut bank so less sand could be removed in storms and to help break the surf action on the dunes at high tide to minimize scour.

Figure 12

Figure 13

Figure 14

Waiting for the Morse River to Shift Course

A photographic flight by the Department of Conservation in November 2009 over Popham Beach State Park provides a different perspective of the tidal inlet, channel course, and dune system. From overhead it is possible to trace out the curved Morse River channel and see where the outer cut banks of the meanders are (Figure 15). Looking upstream to the back-barrier salt marsh system and Seawall Beach it is possible to see how the river flows out from behind the dunes and takes a sharp bend on the ebb-tidal delta and turns toward the state park (Figure 16).

Figure 15

Figure 16

The Morse River has shown signs of starting to cut a new, and more direct, course to the sea across the Seawall Beach spit (Dickson, 2009). The narrowest part of the spit system is directly across from the river mouth where it exits from the back-barrier salt marshes (Figure 17). In November the spit appeared to show signs of being cut by the river (Figure 18). This area appears to have lowered and widened in the same area where downcutting had been previously noted in April (Dickson, 2009 Figure 13, Figure 14, and Figure 15).

Figure 17

Figure 18

The December 12 storm surge resulted in the Morse River ebbing across the spit around the time of high tide, but the scour did not lower the level of sand enough to change the course of the river. It may take a larger storm or a series of severe storms to breach the spit. Alternatively, the river may thin the spit even further as the meandering process continues to lead to channel shoaling and greater sinuosity. A thin spit will have less sand that needs to be removed by a storm and would more likely result in a breach that is permanent. Over time it seems likely that the spit will be breached one way or another and erosion of the dunes at the state park will end for several decades.

Walking the Beach to Observe Coastal Change

A walk on Popham Beach is always a different experience from one day to another and certainly from one year to the next. Keep the historical meandering in mind if you visit the beach and, if visibility is good, look across to the Seawall Beach spit to see if it shows new signs of thinning or lowering and ultimately changing the course of the Morse River. You may be able to see dramatic changes to the coastal geology and ecosystem over several trips to the beach in a single year.

References

Dickson, S. M., 2008a, Tombolo breach at Popham Beach State Park, Phippsburg, Maine.

Dickson, S. M., 2008b, Seawall and Popham Beach Dynamics, Phippsburg, Maine.

Dickson, S.M., 2009, Storm and Channel Dynamics at Popham Beach State Park, Phippsburg, Maine.

Additional Sources of Information

Meandering and other physical properties of rivers

Coastal barrier beach and dune systems of the U.S.

Site by Stephen M. Dickson

Originally published on the web as the January 2010 Site of the Month.

Last updated on April 23, 2012