According to the National Oceanic and Atmospheric Administration, brackish water has a salinity--a measure of dissolved salts--between 0.5 and 35 parts per thousand.
In a coastal estuary, the surge of freshwater moving out of the lower river, the differing densities of fresh and saline water and the cyclical influence of the tides causes the mixing of currents. Because freshwater is less dense than saltwater, the river's freshwater moves toward the sea atop the marine bottom currents coming in. The freshwater layer usually mixes somewhat with the saltiness below to produce a brackish current.
The exact type of estuary circulation varies for a variety of reasons, including seasonal differences in river discharge. For example, as M. Grant Goss describes it in Oceanography, the movement of saltwater up the Columbia River is more substantial during the summer and autumn dry-season when the river is moving slowly. When the Columbia is moving quickly, saltwater wedges only minimally into the river mouth.
Besides river mouths, other bodies of water can be brackish. Two notable examples are the Black and Baltic seas of Eurasia.