Question: The county seat of Suffolk County, NY lies at the mouth of a river that flows into which body of water?

Context paragraphs:
Riverhead (town), New York: Riverhead is a town within Suffolk County, New York, on the north shore of Long Island.  As of the 2010 census, the population was 33,506.  The town rests on the mouth of the Peconic River, from which it derives its name.  Since 1727, Riverhead has been the county seat of Suffolk County.  The smaller hamlet of Riverhead lies within it, and is the town's principal economic center.  The town is 166 miles (267 km) southwest of Boston via the Orient Point-New London Ferry, and is 76 miles (123 km) northeast of New York City.

New York State Route 24: New York State Route 24 (NY 24) is an east–west state highway on Long Island in New York in the United States.  The route is split into two segments, with the longest and westernmost of the two extending from an intersection with Interstate 295 (I-295, named the Clearview Expressway) and NY 25 (Hillside Avenue) in the Queens Village section of the New York City borough of Queens to a junction with NY 110 in East Farmingdale in the Suffolk County town of Babylon.  The shorter eastern section, located in eastern Suffolk County, runs from an interchange with I-495 in Calverton to an intersection with County Route 80 (CR 80) in Hampton Bays.

Big River (Missouri): The Big River is a tributary of the Meramec River in east-central Missouri.  The river rises in western Iron County ( ), near the summit of Johnson Mountain and the locale of Enough; it flows through Washington County, Saint Francois County, and Jefferson County.  It forms part of the boundary between Jefferson County and Saint Francois County and also part of the boundary between Jefferson County and Washington County.  It empties into the Meramec River opposite Eureka ( ), where the Meramec forms the border between Jefferson County and Saint Louis County.  The river flows through Washington State Park, St. Francois State Park, and the Lead Belt mining district.  The elevation of the river at its source is approximately 1300 ft above sea level and at its mouth about 400 ft .  The length of the river is approximately 145 mi , while the airline distance between source and mouth is about 56 mi .  Its watershed area is 955 sqmi .

Saint Joe River: The Saint Joe River (sometimes abbreviated St. Joe River) is a 140 mi long tributary of Coeur d'Alene Lake in northern Idaho.  Beginning at an elevation of 6487 ft in the Northern Bitterroot Range of eastern Shoshone County, it flows generally west through the Saint Joe River Valley and the communities of Avery and Calder.  Past Calder, it flows into Benewah County and through the town of St. Maries, where it receives its largest tributary, the Saint Maries River.  It then turns northwest, passing through Heyburn State Park before reaching its mouth just north of the Kootenai County line.  Much of the river's route through Heyburn State Park is partially flooded due to raised water levels from Post Falls Dam on the Spokane River below Coeur d'Alene Lake.  With a mouth elevation of 2129 ft , it has been described as the highest navigable river in the world.

Peconic River: The Peconic River is a river within Suffolk County on Long Island, New York in the United States.  The river is located in the eastern end of Long Island.  The Peconic River drains an area between the Harbor Hill and Ronkonkoma terminal moraines, and flows into Flanders Bay which in turn connects to Peconic Bay east of Riverhead.

New York State Route 108: New York State Route 108 (NY 108) is a 1.72 mi north–south state highway located on the Suffolk County side of the Suffolk–Nassau county line on Long Island, New York, in the United States.  It is a spur route connecting NY 25A in Cold Spring Harbor to the Cold Spring Harbor station on the Long Island Rail Road's Port Jefferson Branch via Harbor Road.  Harbor Road terminates at an intersection with Woodbury Road, on the Nassau County line, which carries County Route 11 to the east and unsigned County Route 12 to the west.  NY 108, assigned in the early 1930s, is the shortest state highway on Long Island.

New York State Route 454: New York State Route 454 (NY 454), also known as the Suffolk County Veterans Memorial Highway or simply Vets Highway, is a 13.67 mi east–west divided highway in western and central Suffolk County on Long Island in New York.  It spans from NY 25 (Jericho Turnpike) in Commack to NY 27 (Sunrise Highway) in Holbrook.  The route provides access to the Long Island MacArthur Airport, as well as New York State and Suffolk County government offices, and at one time the Long Island Arena.  NY 454 serves as the northern terminus for the Northern State Parkway in Hauppauge, where a concurrency with NY 347 begins.

New York State Route 347: New York State Route 347 (NY 347) is an east–west state highway located in Suffolk County, New York, in the United States.  It connects the Northern State Parkway in Hauppauge to NY 25A in Mount Sinai.  The route serves as a southern bypass of Smithtown and as a direct link between Nesconset and Port Jefferson.  Along the way, NY 347 intersects NY 25 in Nesconset and Nicolls Road (County Route 97 or CR 97) in Lake Grove.  The westernmost 2 mi of NY 347 is concurrent with NY 454 while the portion northeast of NY 25 parallels NY 25A, which follows a more northerly alignment through the town of Brookhaven than NY 347.

New York State Route 111: New York State Route 111 (NY 111) is a state highway located in Suffolk County, New York, in the United States.  It runs north–south for 9.42 mi through the towns of Islip and Smithtown, connecting NY 27A in the town seat of Islip to the east end of the NY 25 and NY 25A concurrency in Smithtown's Village of the Branch.  Most of the road is a two-lane highway, save for several short four-lane stretches in the vicinity of interchanges along the route.  NY 111 was assigned as part of the 1930 renumbering of state highways in New York as an East Islip–Village of the Branch highway, using what is now County Route 17 (CR 17) south of Hauppauge.  It was realigned to serve the hamlet of Islip in 1966.

Suffolk County, Massachusetts: Suffolk County is a county in the Commonwealth of Massachusetts, in the United States.  As of 2016, the population was 784,230 making it the fourth-most populous county in Massachusetts.  The county seat is Boston, the state capital and largest city.  The county government was abolished in late 1999, and so Suffolk County today functions only as an administrative subdivision of state government and a set of communities grouped together for some statistical purposes.

Supporting paragraphs: Riverhead (town), New York Peconic River
Answer: Flanders Bay