Rivers frequently flow into lakes, a connection that forms a basic part of the Earth’s hydrological cycle. A lake is a large body of water surrounded by land, functioning as a temporary reservoir within the system. The interaction between these two bodies of water is governed by gravity and topography, ensuring a constant flow of water from higher elevations to lower ones.
The Mechanism of River Inflow
The driving force behind a river flowing into a lake is gravity, which pulls water downhill across a landscape. A lake sits at the lowest topographical point within its specific drainage basin, also known as a watershed. This watershed is the entire area of land where precipitation and surface water eventually drain into the lake.
Rivers and streams that discharge into a lake are called inlets or feeder streams, following the downward slope of the land toward this low point. The lake acts as a local base level for all the incoming water, concentrating the flow from the surrounding high-elevation terrain. When the river water enters the lake, it may not immediately mix with the lake water. Sometimes the inflow forms a distinct current that plunges beneath the surface or flows as an interflow layer, especially if the water is denser due to cooler temperatures or suspended sediment.
The Lake’s Role as a Sediment Basin
When swiftly moving river water enters the relatively still lake, its velocity drops drastically. This abrupt reduction in speed causes the water’s capacity to carry suspended material to diminish instantly. The river had been transporting a significant sediment load, including silt, sand, and clay, which now settles out of the water column.
This process, known as sedimentation, causes the lake to function as a natural sediment trap for the entire watershed. Large particles, like coarser sands and gravel, are deposited first, often forming a delta at the river’s mouth. Finer silts and clays are carried farther into the lake basin before settling onto the lakebed. This continuous deposition, which also includes nutrients and organic debris, gradually fills the lake basin over geological time, a process called lake infilling.
What Happens to the Water Next
Once the water enters the lake, it has two primary fates depending on the geological setting and climate. Most lakes are classified as part of an exorheic system, meaning they have an outlet river that continues the water’s journey toward the sea. The Great Lakes, for instance, are exorheic, with their water eventually flowing out via the St. Lawrence River to the Atlantic Ocean.
In these open systems, the water level remains relatively stable because the outflow adjusts to match the inflow, ensuring the continuous movement of freshwater. Conversely, some lakes exist within an endorheic basin, which is a closed system with no outlet river connecting to the ocean. In these terminal lakes, water leaves the system primarily through evaporation or by seeping into the ground.
Endorheic lakes, such as the Great Salt Lake, are common in arid and semi-arid climates where evaporation rates are high. Since minerals and salts brought in by the river remain behind when the water evaporates, these closed lakes tend to become saline over time.