marshy land; bog or fen
A low tract of moist or marshy land.
A long narrow and shallow trough between ridges on a beach, running parallel to the coastline.
Bioswale, a shallow trough dug into the land on contour (horizontally with no slope), whose purpose is to allow water time to percolate into the soil.
A shallow, usually grassy depression sloping downward from a plains upland meadow or level vegetated ridgetop.
A gutter in a candle.
A shallow troughlike depression that's created to carry water during rainstorms or snow melts; a drainage ditch.