The difference between saltwater and freshwater lies in the concentration of dissolved salts, known as salinity. Salinity is expressed in parts per thousand (PPT). Water is classified as freshwater when its salinity is less than 0.5 PPT of total dissolved solids. The world’s oceans are saltwater, maintaining an average salinity of approximately 35 PPT. Rivers, for the vast majority of their length, belong to the freshwater category.
The Primary Role of Rivers in the Water Cycle
Rivers serve a primary function as conduits in the global hydrologic cycle, channeling water across the land’s surface. The water that forms a river originates almost entirely from atmospheric moisture that has fallen as precipitation, such as rain or snow. This recently condensed water is exceptionally pure, having left any dissolved solids behind when it evaporated from the ocean or land surface. Rivers act as the major drainage system, collecting this water from surface runoff and also from groundwater that infiltrates the soil and rock layers.
The entire area that drains into a river is known as its watershed or drainage basin. Water within this system is continually in motion, flowing under the influence of gravity from higher elevations to lower ones. This rapid cycling of water, moving constantly from the atmosphere, over the land, and back toward the ocean, is why river water differs so markedly from the accumulated salts of the ocean.
The Standard Salinity of River Water
The water in rivers is overwhelmingly classified as freshwater, consistently measuring below the 0.5 PPT threshold. Although rivers are not chemically pure, they contain a very low concentration of dissolved minerals and salts. Even though the water starts as pure precipitation, it immediately begins to acquire these dissolved solids as it flows over and through the land.
The water’s journey causes it to dissolve small amounts of soluble minerals from the rocks and soil it contacts, a process known as chemical weathering. These minerals include salts like sodium chloride, as well as calcium and magnesium carbonates. The factor preventing the water from becoming saline is the continuous flow of the river toward the sea. As the river constantly transports this mineral-laden water downstream, it prevents the salts from concentrating in one location.
River water contains only about 0.01 to 0.05 PPT of dissolved solids, a tiny fraction of the ocean’s salinity. The continuous transport of this mineral-laden water downstream ensures it remains dilute throughout the river’s course. The salts carried by all the world’s rivers are ultimately delivered to the sea, where they accumulate over eons, explaining why the ocean is saline.
Transition Zones Where Salinity Changes
While rivers are freshwater for most of their length, there are specific transition zones where their salinity changes dramatically. The most common of these zones is the estuary, which is a partially enclosed coastal body of water where a river meets the sea. Here, the fresh river water outflow mixes with the incoming, dense ocean saltwater, creating a zone of brackish water with salinity levels ranging from 0.5 to 31 PPT. The salinity in an estuary constantly fluctuates, varying with the strength of the river’s discharge, tidal cycles, and the specific location within the mixing zone.
In tidal rivers, a phenomenon called a salt wedge can occur, where the denser ocean water travels along the riverbed beneath the lighter freshwater, sometimes far inland. This wedge is pushed and pulled by the tides, causing temporary salinity fluctuations that can affect aquatic life. Another exception occurs in endorheic basins, where rivers flow into landlocked lakes instead of reaching the sea. Since there is no outlet, all the water that enters leaves only through evaporation.
When the water evaporates, the minerals and salts it carried are left behind, accumulating over time. This results in hyper-saline bodies of water, such as the Great Salt Lake or the Dead Sea, which can be many times saltier than the ocean. In these isolated systems, the lower reaches of the inflowing rivers cease to be freshwater at their terminus.