This is a copy of all the feature classes and their definitions found on the GNIS website.

Feature Class Definition
Airport Manmade facility maintained for the use of aircraft (airfield, airstrip, landing field, landing strip).
Arch Natural arch-like opening in a rock mass (bridge, natural bridge, sea arch).
Area Any one of several areally extensive natural features not included in other categories (badlands, barren, delta, fan, garden).
Arroyo Watercourse or channel through which water may occasionally flow (coulee, draw, gully, wash).
Bar Natural accumulation of sand, gravel, or alluvium forming an underwater or exposed embankment (ledge, reef, sandbar, shoal, spit).
Basin Natural depression or relatively low area enclosed by higher land (amphitheater, cirque, pit, sink).
Bay Indentation of a coastline or shoreline enclosing a part of a body of water; a body of water partly surrounded by land (arm, bight, cove, estuary, gulf, inlet, sound).
Beach The sloping shore along a body of water that is washed by waves or tides and is usually covered by sand or gravel (coast, shore, strand).
Bench Area of relatively level land on the flank of an elevation such as a hill, ridge, or mountain where the slope of the land rises on one side and descends on the opposite side (level).
Bend Curve in the course of a stream and (or) the land within the curve; a curve in a linear body of water (bottom, loop, meander).
Bridge Manmade structure carrying a trail, road, or other transportation system across a body of water or depression (causeway, overpass, trestle).
Building A manmade structure with walls and a roof for protection of people and (or) materials, but not including church, hospital, or school.
Canal Manmade waterway used by watercraft or for drainage, irrigation, mining, or water power (ditch, lateal).
Cape Projection of land extending into a body of water (lea, neck, peninsula, point).
Cave Natural underground passageway or chamber, or a hollowed out cavity in the side of a cliff (cavern, grotto).
Cemetery A place or area for burying the dead (burial, burying ground, grave, memorial garden).
Census A statistical area delineated locally specifically for the tabulation of Census Bureau data (census designated place, census county division, unorganized territory, various types of American Indian/Alaska Native statistical areas). See also Civil and Populated Place classes.
Channel Linear deep part of a body of water through which the main volume of water flows and is frequently used as aroute for watercraft (passage, reach, strait, thoroughfare, throughfare).
Church Building used for religious worship (chapel, mosque, synagogue, tabernacle, temple).
Civil A political division formed for administrative purposes (borough, county, incorporated place, municipio, parish, town, township) with legally defined boundaries. See also Census and Populated Place classes.
Cliff Very steep or vertical slope (bluff, crag, head, headland, nose, palisades, precipice, promontory, rim, rimrock).
Crater Circular-shaped depression at the summit of a volcanic cone or one on the surface of the land caused by the impact of a meteorite; a manmade depression caused by an explosion (caldera, lua).
Crossing A place where two or more routes of transportation form a junction or intersection (overpass, underpass).
Dam Water barrier or embankment built across the course of a stream or into a body of water to control and (or) impound the flow of water (breakwater, dike, jetty).
Falls Perpendicular or very steep fall of water in the course of a stream (cascade, cataract, waterfall).
Flat Relative level area within a region of greater relief (clearing, glade, playa).
Forest Bounded area of woods, forest, or grassland under the administration of a political agency (see "woods") (national forest, national grasslands, State forest).
Gap Low point or opening between hills or mountains or in a ridge or mountain range (col, notch, pass, saddle, water gap, wind gap).
Glacier Body or stream of ice moving outward and downslope from an area of accumulation; an area of relatively permanent snow or ice on the top or side of a mountain or mountainous area (icefield, ice patch, snow patch).
Gut Relatively small coastal waterway connecting larger bodies of water or other waterways (creek, inlet, slough).
Harbor Sheltered area of water where ships or other watercraft can anchor or dock (hono, port, roads, roadstead).
Hospital Building where the sick or injured may receive medical or surgical attention (infirmary).
Island Area of dry or relatively dry land surrounded by water or low wetland (archipelago, atoll, cay, hammock, hummock, isla, isle, key, moku, rock).
Isthmus Narrow section of land in a body of water connecting two larger land areas.
Lake Natural body of inland water (backwater, lac, lagoon, laguna, pond, pool, resaca, waterhole).
Lava Formations resulting from the consolidation of molten rock on the surface of the Earth (kepula, lava flow).
Levee Natural or manmade embankment flanking a stream (bank, berm).
Locale Place at which there is or was human activity; it does not include populated places, mines, and dams (battlefield, crossroad, camp, farm, landing, railroad siding, ranch, ruins, site, station, windmill).
Military Place or facility used for various aspects of or relating to military activity.
Mine Place or area from which commercial minerals are or were removed from the Earth; not including oilfield (pit, quarry, shaft).
Oilfield Area where petroleum is or was removed from the Earth.
Park Place or area set aside for recreation or preservation of a cultural or natural resource and under some form of government administration; not including National or State forests or Reserves (national historical landmark, national park, State park, wilderness area).
Pillar Vertical, standing, often spire-shaped, natural rock formation (chimney, monument, pinnacle, pohaku, rock tower).
Plain A region of general uniform slope, comparatively level and of considerable extent (grassland, highland, kula, plateau, upland).
Populated Place Place or area with clustered or scattered buildings and a permanent human population (city, settlement, town, village). A Populated Place is not incorporated and has no legal boundaries. See also Census and Civil classes.
Post Office An official facility of the U.S. Postal Service used for processing and distributing mail and other postal material.
Range Chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra).
Rapids Fast-flowing section of a stream, often shallow and with exposed rock or boulders (riffle, ripple).
Reserve A tract of land set aside for a specific use (does not include forests, civil divisions, parks).
Reservoir Artificially impounded body of water (lake, tank).
Ridge Elevation with a narrow, elongated crest which can be part of a hill or mountain (crest, cuesta, escarpment, hogback, lae, rim, spur).
School Building or group of buildings used as an institution for study, teaching, and learning (academy, college, high school, university).
Sea Large body of salt water (gulf, ocean).
Slope A gently inclined part of the Earth's surface (grade, pitch).
Spring Place where underground water flows naturally to the surface of the Earth (geyser, seep).
Stream Linear body of water flowing on the Earth's surface (anabranch, awawa, bayou, branch, brook, creek, distributary, fork, kill, pup, rio, river, run, slough).
Summit Prominent elevation rising above the surrounding level of the Earth's surface; does not include pillars, ridges, or ranges (ahu, berg, bald, butte, cerro, colina, cone, cumbre, dome, head, hill, horn, knob, knoll, mauna, mesa, mesita, mound, mount, mountain, peak, puu, rock, sugarloaf, table, volcano).
Swamp Poorly drained wetland, fresh or saltwater, wooded or grassy, possibly covered with open water (bog, cienega, marais, marsh, pocosin).
Tower A manmade structure, higher than its diameter, generally used for observation, storage, or electronic transmission.
Trail Route for passage from one point to another; does not include roads or highways (jeep trail, path, ski trail).
Tunnel Linear underground passageway open at both ends.
Unknown This class is assigned to legacy data only. It will not be assigned to new or edited records.
Valley Linear depression in the Earth's surface that generally slopes from one end to the other (barranca, canyon, chasm, cove, draw, glen, gorge, gulch, gulf, hollow, ravine).
Well Manmade shaft or hole in the Earth's surface used to obtain fluid or gaseous materials.
Woods Small area covered with a dense growth of trees; does not include an area of trees under the administration of a political agency (see "forest").

They use the word slough as a synonym under both gut and stream.

The Wikipedia definition of a slough is:

On the northern Great Plains of the United State, a slough is a pond (often alkaline) usually the result of glaciation (see kettle (geology)); also called a pothole, whence Prairie Pothole Region to describe the area where these sloughs are abundant.  On the Canadian Prairies, slough refers to any naturally formed shallow fresh water (or alkaline) pond, usually habitat for waterfowl, synonymous with the term marsh. Slough may also refer to any naturally occurring body of open water smaller than a lake. Sloughy (slew-ee) refers to any murky, muddy body of water, as in a "sloughy lake", a lake unsuitable for swimming.

The Canadian Prairies definition highlighted above is almost always used here.  They could be called marshes or swamps, but we always call them sloughs. 

There are thousands of sloughs in this area and many of the named lakes could be defined as sloughs, and many of the sloughs could be defined as lakes.

See also:

GNIS database

GNIS home page