The Wagging Tail of the Merrimack River

February 21, 2016 by Chris Hein

Williamsburg, VA       21 February 2016

Picking up where I left off yesterday . . .

So, I’ve shared some of my perspective about the need for sound science to inform management of northern Plum Island and the erosion problem at Reservation Terrace. It has been suggested by some residents and some in the local media that removal of the jetties (or at least the south jetty) will remedy this situation and/or help the erosion problem along Annapolis/Fordham Way. As part of our study of Plum Island, we’ve looked into just this. Well, not directly . . .

One of the key tenants in geology is that the present is the key to the past. Put another way, the past (that is, what we can learn from the historic and geologic / sediment records) is the key to the future. What does that have to do with the erosion at Plum Island Point? Quite a bit.

If you look at historic maps of northern Plum Island, you’ll see that it has not always looked like it does, with two “forks” separated by the Basin. In fact, the entire northeast end of Plum Island is ephemeral over 10s to 100-year timescales: it builds, gets eroded away, rebuilds, and so on. Prior to the installation of the jetties to stabilize the mouth of the Merrimack River, this happened at least twice since Europeans settled the lower Merrimack Valley. Check out the maps (and my annotated interpretations) below.

Historic maps (top) and annotated diagrams (bottom) of the changes at northern Plum Island from 1741 to 1883 (installation of the jetties).

Historic maps (top) and annotated diagrams (bottom) of the changes at northern Plum Island from 1741 to 1883 (installation of the jetties).

 

Over time, the Merrimack River, like most un-jettied tidal inlets, “wags”, like the end of the tail of a dog (or cat, if you prefer felines [I do]). Why? Well, it comes down to the two big forces along the coast: tides and waves. Tides work cross-shore, moving water and sediment through breaks in the beach / barrier islands (tidal inlets) between the ocean (the Gulf of Maine in this case) and the “backbarrier” (the marshes, tidal flats, lagoons, and estuaries landward of the barrier island; the Merrimack River mouth in this case). Tidal forces want to move water back and forth and quickly and efficiently as possible. The fastest route between the backbarrier and the ocean is the same as the shortest distance between two points: a straight line. In the case of northern Plum Island, that means that the Merrimack River would go almost straight east-west.

The other major force along the coast is waves. Well, waves just come from offshore and strike the beach head-on right? So they are moving water (and therefore the energy to move sand around) in the same direction as tides (cross-shore), right? Not quite. Recall any time you’ve been swimming at the beach for a while and notice that when you come out of the water, you have been moved down along the beach (parallel to the beach) from where you left your towel. That is a process called “longshore transport” and it is due to the fact that waves do not strike the beach straight-on (in most cases), but instead arrive from a slight angle to the beach. They tend to straighten out as they approach the shore and break, but they still retain a bit of an angle . . . enough to move you, or sand, along the shore.

Over longer periods of time (months to years or longer), sand grain by sand grain, this can move large amounts of sand, and alter the shape of the coast. Sand gets trapped on the “downdrift” side of groins (the side from which waves are coming) and it gets moved far down the beach (this is why when one person uses hard structures to trap sand in front of their beach, or prevent erosion of a bluff, someone further “downdrift” is being robbed of sand that doesn’t make it to them, and they start to experience erosion). Mike Morris covers this topic very well on his website, with a lot of nice graphics: http://www.plumislanderosion.com/

Although Mike and I disagree on the importance of southerly swell (waves from the southeast, roughly) along particular sections of the Center Island to Annapolis Way sections of Plum Island, we do agree that longshore transport plays a major role in the changes we see along this beach. We also agree that, over long periods of time (decades to centuries to millennia) waves from the northeast play a dominant role in shaping the island; that is why Plum Island has grown as a spit to the south over the last 4000 years. Southerly longshore transport by waves from the northeast (dominantly during our winter nor’easters which have some of the biggest waves during the year) is also how Cranes & Wingaersheek Beaches obtained their sand, which, by the way, is all from the Merrimack River.

Ok, so what does this all mean for the mouth of the Merrimack River? Well, those tidal forces are working to keep it as straight as possible: east-west. Waves are working to shift its position. Over time periods of decades, that shift is always to the south: the mouth is literally pushed to the south as the spit of Salisbury Beach grew to the south. This, of course, causes the Merrimack River Inlet to impinge upon northeastern Plum Island, causing massive erosion. How massive? Well, prior to those jetties, the southwesterly migration of the Merrimack River mouth wiped out the entire northeast fork of Plum Island (called “Old Point”) in the early 1800s. It eroded down to roughly the position of 16th Street. Nothing north nor east of there existed (those areas were all open water). As late as ~1840, the Merrimack River ran right through the Basin and under where Mad Martha’s (yum! by the way) sits today.

That’s right, the Basin is the old channel of the Merrimack River.

So, what happened? Well, eventually, the Merrimack River got too far southeast: those tidal forces, which work to give the river the shortest route to the ocean, became too strong for the wave forces. The Merrimack River blew through southern Salisbury (possibly during a big storm) and created its current channel. This is a very common process at tidal inlets; it is called “ebb-delta breaching” because the river/inlet cuts through its ebb-tidal delta (that area of shallow water offshore of the river mouth we now call “The Bar” or “The Breakers”) to give itself the shortest route possible to the ocean.

After this breaching event, a large bar of sand (which had been the ebb tidal delta of the Merrimack before the breaching event) was free to move, and it migrated onshore, eventually building up to become the northeast fork of Plum Island. Some of the former river channel was filled (hence our ability to go to enjoy those awesome omelets at Martha’s), but the rest of the channel was left behind as a long, narrow body of water that still looks like the river channel it once was: The Basin.

Soon after this last breaching event, the US Army Corps built the jetties and stabilized the mouth of the Merrimack and allowing for further growth of “New Point” (the northeast fork, including, of course, Plum Island Point which is today eroding). That put an end to the natural “wagging” of the Merrimack River, though started a pattern of erosion that comes and goes along the Newbury portion of Plum Island beach and at Plum Island Point . . . that part is a story for another day.

Using the tools of sedimentology and geology, we went back to this area to check and see if we could find evidence of this series of events. And did we ever! Using a tool called “ground-penetrating radar” (GPR) which sends a radar signal into the ground from a little antenna and allows scientists to “see” into the subsurface, we imaged the old river channel under 16th St. The series of dipping lines you see in my annotated figure below (with the actual data above it) is a bar of the Merrimack River channel growing as the channel migrated. We took a sediment core through this with our Geoprobe Drill Rig back in May 2014 . . . the pebbles and gravel we see at the bottom of that bar are from the Merrimack River bottom – this confirms that we we found is indeed the early 1800s Merrimack River.

Ground-penetrating radar profile of the former Merrimack River channel, as imaged under 16th Street. The section shown in blue is the former channel. The elevations to the left are in meters with respect to mean sea level. On the right the “depth” in the profile is given in TWTT, or two-way travel time.

 

I’ll be presenting our findings to a large international meeting of coastal scientists in Australia in a few weeks. As part of this study we compared what we know about the Merrimack River ebb-delta breaching from the historic maps and what we can see in GPR and sediment cores to the GPR and sediment signatures of a 3600-year-old tidal inlet I discovered in central Plum Island back in 2012. One inlet is 150 years old and associated with the Merrimack River . . . the other is 3600 years old and associated with the Parker River when it once shot straight through Plum Island and out to the Gulf of Maine. Cool . . . at least I think so.

I posted a copy of the scientific poster I made for this meeting here. Check it out and let me know (hein@vims.edu) if you have any questions.

On last interesting facet of this study is that this series of events I’ve described which formed The Basin and Plum Island Point is not new. In fact, this likely happened many times in the past. Check out those maps above again: Old Point was only starting to form in 1741, probably very soon after the previous ebb-delta breaching event and re-orientation of the Merrimack River mouth. We don’t have data prior to the early 1700s, but I would guess this happened over and over, probably once every 50-100 years or so.

If you want an example of how this STILL occurs, just check out a “natural” inlet (one with no jetties). Time Magazine’s Timelapse (http://world.time.com/timelapse/) is a great tool to view 30 years of satellite imagery: you can really watch the coast change before your eyes. Just click on the “Explore the World” link on the bottom right of the main image on that page and then type in “Chatham, MA” and hit enter. That is not ebb-delta breaching, but you’ll get a great sense of just how dynamic the interaction between waves and tides at natural tidal inlets can be.

I know this is a dense post, but we’re all very excited by these results and wanted to share them with our readers of this blog. We’re at the point in this project where we have results and conclusions coming hot and fast and we look forward to posting more of them soon, so check back. In the meantime, I hope this all made sense and hopefully sheds some light on just what northern Plum Island would be doing if not for those jetties . . . and therefore maybe add a bit more science, as learned from the history of Plum Island, to the current debate over how to address the severe erosion at Plum Island Point.

— Chris

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