Rivers are generally considered fresh water, which means they contain a low concentration of dissolved salts. For water to be classified as fresh, the total dissolved salt content must be less than 1,000 parts per million (ppm), or less than 0.1% salt by weight. By contrast, salt water, such as that found in the ocean, has an average salinity of about 35,000 ppm, or 3.5%. Rivers do contain trace amounts of dissolved minerals, but their low concentration places them firmly in the fresh water category. This fundamental difference in salinity dictates the types of ecosystems that can thrive in a river environment versus a marine environment.
The Constant Cycle of Fresh Water
The primary reason rivers remain fresh is the continuous cleansing action of the hydrological cycle. River water originates as precipitation, which is created when water vapor condenses in the atmosphere. When water evaporates from the Earth’s surface, the salt and other dissolved solids are left behind, effectively distilling the water.
This atmospheric distillation process results in precipitation that is nearly pure H₂O, with total dissolved solids (TDS) often less than 20 to 50 ppm. Rain and snowmelt constantly replenish the rivers with this highly pure water. This immense volume of pure water dilutes any minerals the river picks up, preventing high concentrations.
This mechanism contrasts with the ocean, which acts as a vast terminal reservoir. While the ocean continually loses pure water through evaporation, the dissolved salts remain behind, concentrating the salinity over geological timescales. Since most rivers flow toward the ocean, the freshwater supply is constantly renewed and flushed, unlike the static, accumulating nature of the sea.
How Rivers Acquire Dissolved Minerals
Although rivers are fresh, they are not composed of pure water; they contain a variety of dissolved minerals. The process through which these minerals enter the water is called chemical weathering, which begins as soon as the water touches the land. As rainwater flows over rocks and soil, it acts as a weak acid, dissolving small amounts of mineral content.
This dissolution extracts ions like calcium, magnesium, sodium, and chloride from the surrounding geology. The flowing river water then transports these dissolved solids downstream as solutes. This process explains why even fresh river water has a noticeable taste and contributes to the overall health of the river ecosystem.
Over millions of years, the cumulative effect of rivers washing these minerals into the sea has made the ocean salty. The river’s role is to act as a transporter, continuously moving trace amounts of dissolved rock from the land to the ocean. This constant delivery of solutes is the source of all the salt in the world’s oceans.
Understanding Estuaries and Tidal Mixing
The most common exception to the rule of rivers being fresh water occurs in estuaries, which are transitional zones where a river meets the sea. In this environment, the river water and the ocean water mix, creating brackish water that has a salinity between 0.5% and 3%. The extent of salt water in a river’s mouth depends heavily on the dynamics between river flow and tidal action.
Because ocean water is denser due to its high salt content, it often flows beneath the lighter fresh water from the river. This creates a salinity gradient where a wedge of salt water can intrude far upstream along the riverbed. Tides powerfully drive this intrusion, pushing the salt water upriver during high tide, a process known as tidal mixing.
The distance this salt water travels inland is determined by the river’s discharge rate; slower flows allow the salty wedge to penetrate much further upstream. Estuaries are biologically important because their fluctuating, brackish conditions support unique and highly adapted plant and animal species.
Rivers in Closed Basins
A less common, non-coastal exception involves rivers that flow into endorheic, or closed, basins. These are areas where the river does not drain into the ocean but instead terminates in an inland lake or basin. In these systems, water only leaves the basin through evaporation, which fundamentally alters the water’s chemistry.
While the river water initially brings dissolved minerals into the lake, the evaporation process removes the pure water, leaving the minerals behind to concentrate. Over time, this leads to the water body becoming hypersaline, sometimes reaching salt concentrations far higher than the ocean, as seen in the Great Salt Lake or the Dead Sea. Additionally, some rivers can become locally saline when they flow through geological formations that contain high levels of salt-bearing rock strata, such as in the Pecos River basin.