The Coastal Lagoons of Southern Rhode Island
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Coastal Lakes The Coastal Lagoons of Southern Rhode Island Elise Torello and Rhonda Smith dorning the beautiful southern development and golf courses. Next in Cards Pond, also very small and usually coastline of Rhode Island like line are Winnapaug, Quonochontaug, separated from the ocean, is periodically Ajewels are about a dozen special and and Ninigret Ponds, all of which have breached manually to control water levels. surprisingly diverse water bodies known permanent, hardened connections to the The last coastal lagoon to the east is Potter collectively as “the salt ponds.” In truth, ocean (breachways) constructed in the Pond, which unlike the other coastal none of these water bodies are actually middle of the 20th century. Next is Green lagoons that are oriented parallel to the ponds; most are more accurately referred Hill Pond, surrounded and impacted coast, is oriented more perpendicular to to as “coastal lagoons,” and one is an by development and without its own the coast. The northern basin of Potter estuary (Figure 1). breachway. Green Hill Pond does have Pond is a deep “kettle hole” created as It is surprising that in a mere 20-mile limited water exchange with neighboring the last glacier retreated. Potter’s only stretch of coastline, from Watch Hill in Ninigret Pond via a narrow channel. water exchange with Block Island Sound the west to Point Judith in the east, each Trustom Pond, to the east of Green Hill is via a channel into adjacent Point Judith of these water bodies can be so unique. Pond, is a fascinating, ongoing case study Pond, which is actually the estuary of the Furthest to the west among the largest ten in the dynamic nature of coastal lagoons. Saugatucket River. Point Judith Pond is of these lagoons are tiny Maschaug and Usually separated from the ocean by the home to the Port of Galilee, which is one Little Maschaug Ponds, usually separated Moonstone Barrier, Trustom Pond was a of the largest fishing ports on the east from the ocean by their coastal barrier freshwater system until it was breached coast. and highly impacted by surrounding during extra-tropical storm Sandy. Figure 1. The coastal lagoons of southern Rhode Island. Winter 2013 / LAKELINE 21 The southern Rhode Island coastal melted enough glacial ice to bring the sea processes cause coastal barriers to move lagoons were formed after the retreat of level up to its approximate current height by landward and upward. the Laurentide Ice Sheet at the end of the about 4,000 years ago (CRMC 1999). With people living and building last glaciation. The Laurentide Ice Sheet With wind, waves, and time, glacial businesses around the ponds and on the extended south to Nantucket, MA, Block deposits were eroded and carried along the coastal barriers, the natural processes driving Island, RI, and Long Island, NY, about shoreline by currents (Figure 2). This eroded the configuration and migration of these 25,000 to 30,000 years ago (CRMC 1999). material formed barrier spits between the features no longer proceed unfettered. For At that time, with so much water locked up higher headlands. Continuing erosion and many decades, overwashed sand that landed in ice sheets, the coastline was almost 100 deposition of sediments by waves and long- on roads or developed property has been miles further south than it is now and the sea shore currents caused the spits to grow away moved back to the beach face, interfering level was about 120 meters lower. The thick from the headlands parallel to the shoreline. with the natural landward migration of glacial ice carried with it material ranging Eventually the spits almost completely the coastal barriers. In addition, ongoing, in size from small particles to rocks and connected the headlands, isolating the decades-long studies of the advance and boulders, collectively referred to as glacial coastal lagoons behind them from the ocean retreat of the southern RI shoreline show till. except for narrow inlets through which that long-term, most of the shoreline is As the climate warmed, the ice ocean tides flowed (CRMC 1999). Unlike eroding (CRMC 1999). Prevention of retreated until the ice margin reached a today, the inlets were not fixed in one place; sand from reaching the back of the coastal position just north of the current RI coastline rather, they periodically closed, changed barriers during and after storms, combined approximately 21,000 years ago. The ice location, and were re-opened by storm with erosion of beach faces, over time will margin fluctuated at this location a time, surges and waves. cause the coastal barriers to narrow. This is forming a hill of glacial material known as Storm waves, storm surges, and rising a problem, since narrow, developed coastal the Charlestown Moraine (Dr. J. Boothroyd, sea level are continuing the process of barriers do not provide as much storm personal communication). Additional sand eroding the glacial deposits. During big protection to landward structures as wider, and gravel was deposited by meltwater storms, storm surges and waves roll over more natural barriers. An accelerating rate of rivers flowing underneath the melting low, narrow barrier spits (overwash). The sea level rise in the northeastern U.S., plus ice sheet, around its edges, and through overwash transports sand from the ocean and the prospect of increasing storm frequency breakout channels along the length of the ice beach face across the barriers and sometimes and severity due to climate change, is margin. These deposits, along with the direct all the way to the lagoons. In severe storms, driving a major policy effort in RI to deposition of till by the ice, created a series temporary storm-surge channels might cut address shoreline change and sea level rise of headlands along the coast. Rapid climate through the barriers, allowing additional adaptation statewide (www.beachsamp.org). warming starting about 14,000 years ago sediment into the lagoon. Over time, these Figure 2. The geologic processes that formed southern Rhode Island’s coastal lagoons. 22 Winter 2013 / LAKELINE Other Issues Facing the Coastal Lagoons Trustom Pond: An Ongoing Case Study very high, which likely contributed to As is true for so many water bodies, in Coastal Lagoon Dynamics the breach being larger than it had been the southern RI coastal lagoons are being Trustom Pond is a 160-acre coastal in previous years. While the breach was impacted by the human activity taking place pond completely within the boundaries of active, the water level in the Pond was at around them. Excess nutrients from septic the U.S. Fish and Wildlife Service’s (US least four feet lower than pre-breach and systems, fertilizers, and animal waste are F&WS) Trustom Pond National Wildlife the salinity averaged 29 ppt (seawater carried into the lagoons by surface runoff Refuge (NWR) in South Kingstown, is about 35 ppt). These are the highest and groundwater. The excess nutrients fuel RI. Trustom Pond is unique in that it is salinity measurements documented in excess algal growth, which causes decreases Rhode Island’s only coastal pond free of Trustom Pond; previous breaching events in water clarity and dissolved oxygen shoreline development. Moonstone Beach, resulted in salinity measurements ranging concentrations. In addition, now that there which is the coastal barrier separating anywhere from 1.7 to 18 ppt. The breach are hardened, permanently fixed breachways Trustom Pond from Block Island Sound, stayed open for five months until closing into several of the lagoons, sand is is managed by the Refuge for the benefit naturally on March 24, 2013 (Figure transported through these inlets by incoming of migratory shorebirds and water birds, 4). As of September 2013, thanks to tides and then deposited within the lagoons including the federally threatened piping the contributions of several freshwater as flood tidal deltas. This buries important plover. tributaries and heavy spring rainfall, the eelgrass (seagrass) and shellfish habitat and From the 1600s to the 1900s, Trustom salinity was down to about 9 ppt and the decreases water circulation in the lagoons. Pond was breached twice annually, water level had risen by several feet. Thus, the hardened breachways must be usually by mechanical methods, to drain The breach event had an immediate maintained through periodic dredging to adjacent agricultural fields and encourage impact on the ecology of Trustom maintain the exchange of pond waters with a healthy shellfish population in the pond. Pond. In past surveys, fish communities clean seawater and prevent shoaling within The Trustom Pond NWR was established consisted of a variety of fresh- and the ponds. in 1974 and from 1975-1996 the pond was brackish-water species, depending The coastal ponds of southern RI, and breached about once per year, primarily to on how recently the pond had been even different locations within a single pond, provide foraging habitat for piping plovers breached. Immediately prior to the 2012 are affected by these stressors to varying and other shorebirds. After 1996, a more breach the fish community consisted degrees, depending upon how densely natural breaching regime was allowed to mainly of common freshwater fish such developed the shore is and how close to take over and the pond was mechanically as largemouth bass and pumpkinseed a breachway a location happens to be. breached only once, in 2006. Mechanical sunfish. Soon after the breach, many dead Outreach efforts are ongoing to encourage breaching done by the Refuge typically fish of these and other freshwater species people who live near the ponds to limit resulted in the pond being open to the were scattered on the mudflats around or eliminate their inorganic fertilizer use, flow of seawater for a period of less than a the pond, as they cannot survive in salt properly maintain their septic systems, pick day to as many as a few days.