| Narragansett Bay | |
|---|---|
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  Narragansett Bay | |
| Location | Rhode Island,Massachusetts, United States |
| Coordinates | 41°36′N71°21′W / 41.600°N 71.350°W /41.600; -71.350 (Narragansett Bay) |
| Type | Bay |
| Surface area | 147 square miles (380 km2) |
Narragansett Bay is a bay and estuary on the north side ofRhode Island Sound covering 147 square miles (380 km2), 120.5 square miles (312 km2) of which is inRhode Island.[1] The bay formsNew England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago.[2] Small parts of the bay extend intoMassachusetts.
There are more than 30 islands in the bay; the three largest ones areAquidneck Island,Conanicut Island, andPrudence Island.[3] Bodies of water that are part of Narragansett Bay include theSakonnet River,Mount Hope Bay, and the southern, tidal part of theTaunton River. The bay opens on Rhode Island Sound and the Atlantic Ocean;Block Island lies less than 20 miles (32 km) southwest of its opening.
"Narragansett" is derived from the southern New EnglandAlgonquian wordNaiaganset meaning "(people) of the small point of land".[4]
The watershed of Narragansett Bay has seven riversub-drainage basins, including theTaunton,Pawtuxet, andBlackstone Rivers, and they provide freshwater input at approximately 2.1 million U.S. gallons (7.9×10^6 L) per day.[5] River water inflow has a seasonal variability, with the highest flow in the spring and the minimum flow in early fall.[6]
The bay is aria estuary (a drowned river valley) which is composed of theSakonnet River valley, the East Passage river valley, and the West Passage river valley. Thebathymetry varies greatly among the three passages, with the average depths of the East, West, and Sakonnet River passages being 121 feet (37 m), 33 feet (10 m), and 25 feet (7.6 m) respectively.[7] Narragansett Bay is aria that consists of a series of flooded river valleys formed of dropped crustal blocks in ahorst andgraben system[8] that is slowly subsiding between a shifting fault system.[9]
Providence isRhode Island's capital and largest city and sits at the northernmost arm of the bay. Many of Providence's suburbs are also on the bay, includingEast Providence,Warwick andCranston.Newport is located at the south end ofAquidneck Island on the ocean, and is the home of theUnited States Naval War College, theNaval Undersea Warfare Center, and a majorUnited States Navy training center. The city ofFall River, Massachusetts is located at the confluence of theTaunton River andMount Hope Bay, which form the northeasternmost part of Narragansett Bay. The southwest side of the bay includes the seaside tourist towns ofNarragansett andWickford.Quonset Point, south of Warwick, gives its name to theQuonset hut.
Tides in the watershed are measured at Providence, Fall River, Quonset Point,Conimicut Light, Prudence Island, and Newport. In shallow water, sound waves are used to measure water height by precisely measuring their travel time. In deeper water, tides are measured with pressure-sensing tide gauges that are placed on the ocean floor to measure water height.
The bay's tides are semi-diurnal, meaning that the region experiences two high and low tides daily. The tides range in height from 3.6 feet (1.1 m) at the bay's mouth and 4.6 feet (1.4 m) at its head. Water depth varies about 4 feet (1.2 m) between high and low tide. The lunar, semi-diurnal M2 tide occurs at a period of 12.42 hours, with two tides occurring in the watershed every 24 hours and 50 minutes. The watershed'sneap and spring tides occur every 14.8 days. In Narragansett bay, the tides show a distinct double-peak flood during high tide and single peak ebb during low tide.[10]
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The movement of water within a system is itscirculation. Estuaries are given a classification depending on the pattern of their circulations. Thecirculation classification can be well-mixed, partially mixed, salt wedge, or Fjord-type. For Narragansett, the circulation is mostly well-mixed; however, the Providence River does show some vertical stratification.
Narragansett Bay circulation is made up of forces provided by the winds, tides, and changes in waterdensity within the watershed. Its circulation is the result of the flow of fresh water at the head interacting with salt water at the point where the bay meets the open ocean. Residence time of water due to the circulation of Narragansett Bay is 10–40 days, with an average of 26 days.[7] Tidal mixing is the dominant driver of circulation patterns in the bay, where currents can reach up to 2.5 feet per second. Non-tidal currents such as the flow of low salinity water at the surface out of the bay and high salinity deep water into the bay contribute to a current of about 0.33 feet per second.[7]
Winds also drive circulation patterns in the bay. In the winter, winds come from the northwest, helping move water towards the bay's mouth; in the summer, they are out of the southwest and move water towards the head of the bay. Wind-driven waves of over 4.25 feet (1.30 m) also help mix surface waters.
Density-driven forces are the third factor affecting circulation. Fresh water inflow comes from natural sources such as atmospheric precipitation and inflow from the many rivers that feed into the watershed, and human-made sources such as water treatment plants. Fresh and saltwater mixing results in asalinity range in the bay of 24 parts per thousand (ppt) in the upper Providence River area to 32 ppt at the mouth of the bay.
The bay's currents and circulation patterns greatly influence the sediment deposits within the region. The majority of the sediments within the bay are fine-grained material such as detritus, clay-silt, and sand-silt-clay. Scientists have been able to identify 11 types of sediment that range from coarse gravels to fine silts.[7] the bay's currents deposit fine materials through the harbors of the lower and middle sections of the bay, and the coarse, heavy materials are deposited in the lower areas of the bay where the water velocities are higher.
The firstvisit by Europeans to the bay was probably in the early 16th century. At the time, the area surrounding the bay was inhabited by two different Indian tribes: theNarragansetts occupied the west side of the bay, and theWampanoags lived on the east side, occupying the land east toCape Cod.
It is accepted by most historians that first contact by Europeans was made byGiovanni da Verrazzano, an Italian explorer who entered the bay in his shipLa Dauphine in 1524 after visitingNew York Bay. Verrazzano called the bayRefugio, the "Refuge". It has several entrances, however, and historians can only speculate as to the exact route of his voyage and the location where he laid anchor, along with a corresponding uncertainty over which tribe made contact with him.[11][12] Verrazzano reported that he found clearings and open forests suitable for travel "even by a large army".[13] Dutch navigatorAdriaen Block explored and mapped the bay in 1614, and nearbyBlock Island is named in his honor.
The first permanent Colonial settlement was established in 1636 byRoger Williams, a former member of theMassachusetts Bay Colony. He was friends with NarragansettsachemCanonicus, who provided him with land on which to buildProvidence Plantations. Around the same time, theDutch established a trading post approximately 12 miles (20 km) to the southwest which was under the authority ofNew Amsterdam in New York Bay. In 1643, Williams traveled to England and was granted a charter for theColony of Rhode Island and Providence Plantations.
TheGaspee Affair was an important naval event which moved theThirteen Colonies toward theAmerican Revolution. It occurred in Narragansett Bay in 1772 and involved the capture and burning of the British customs schoonerGaspee. The American victory contributed to the eventual start of the war at theBattles of Lexington and Concord in Massachusetts three years later. The event is celebrated inPawtuxet Village as the Gaspee Days Celebration[14] in June which includes burning the ship in effigy.
Roger Williams and other early colonists named the islands[15]Prudence,Patience,Hope,Despair, andHog. To remember the names, colonial school children often recited the poem: "Patience, Prudence, Hope, and Despair. And the little Hog over there."[16]
Narragansett Bay is divided byConanicut Island into east and west passages. Its depth averages approximately 9.0 meters throughout the bay, but it is shallower in the west passage and deeper in the east.[17] Narragansett Bay receives freshwater input from several sources, including rivers (approximately 80%), direct precipitation (13%), wastewater treatment facilities (9%), and unknown amounts from ground-water and combined sewer overflows (CSOs).[17] The freshwater input varies greatly from season to season as it is highest in winter and lowest in summer, while CSOs increase dramatically following heavy rain. Water from these freshwater inputs mixes with sea water to form a salinity gradient ranging from 24 ppt at the mouth of theProvidence River to 32 ppt at the mouth of the bay.[17] Water temperatures range between -0.5 °C and 24 °C over the year. Salinity and temperature patterns measured throughout the bay indicate a range of mixed to stratified conditions[18] and the amount of time it takes for the total volume of fresh water in the bay to be equaled by the fresh water input (flushing time) varies from 40 days to 10 days, depending on freshwater input.[19]
Dissolved oxygen levels in Narragansett Bay are seasonal with lower levels in the summer and higher levels in the winter. This is due to warmer temperatures and higher biological demand for oxygen in the summer and the opposite in the winter. Narragansett Bay is also subject to periodic summer hypoxic events in the Northern half of the bay, nearing anoxic conditions in some parts.[20]
Phytoplankton forms the base of the food chain for many of the bay's marine animals. General trends in phytoplankton biomass across the bay are characterized by highest levels in the Providence River and lowest levels inRhode Island Sound, correlating with the North to South nutrient concentration gradient in the bay (24). Near the Providence River, biomass levels are highest in the summer and lowest in the winter. In the east and west passages there are peaks in the spring and fall.[21]
Annual blooms have shrunk over the last decade (2006 onward) as the amount ofnitrogen (an important nutrient) supply to the bay has decreased by more than 50% due to wastewater treatment facilities switching to a new treatment that involves removal of nitrogen[22]
Narragansett Bay is the site of one of the world's longest-running plankton surveys, extending from 1957 through the present. Researchers at theUniversity of Rhode Island’s Graduate School of Oceanography collect samples weekly and the dataset contains 246 different species.[23]
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