Rivers frequently connect to lakes, which are rarely isolated features on the landscape. Instead, they function as integral components within the larger surface water network known as a drainage basin or watershed. The flow of water across a continent follows the principles of hydrology, where gravity drives water from higher elevations toward the sea. This continuous movement links streams, rivers, and lakes into an interconnected system, ensuring the passage of water, sediments, and nutrients throughout the entire network.
Understanding River and Lake Connections
Rivers and lakes typically connect in one of three primary configurations, dictating the flow of water through the system. The most common connection involves the river acting as an inlet, or tributary, that flows into the lake, supplying it with water from an upstream source. This inflow carries the river’s discharge and material load directly into the standing body of water.
The lake maintains a surface elevation until the water finds a point of discharge, forming an outlet river that continues the journey downstream. In this scenario, the lake acts as a substantial storage basin situated directly along the river’s course. The entire path, from the river entering the lake to the river exiting, is known as a through-flow system.
The distinction between upstream and downstream is based on the lake’s position within the watershed. Water entering the lake is considered upstream and water leaving is downstream. The lake’s presence represents a temporary pause in the river’s flow, significantly impacting the river’s characteristics before the water continues onward. This interconnectedness means that changes in the lake environment directly influence the river ecosystem that follows.
How Lakes Change River Characteristics
A lake acts as a significant modifier of the river’s physical and chemical properties, fundamentally changing the nature of the water that flows out of it. This modification occurs through flow regulation, sediment trapping, and alterations to thermal and chemical profiles.
Flow Regulation
The large volume of the lake acts as a natural reservoir, providing flow regulation. The lake absorbs large influxes of water from heavy rainfall or snowmelt, buffering the flow and reducing the peak intensity of potential floods downstream. During periods of drought or low precipitation, this stored water is released slowly through the outlet river, helping to maintain a more consistent flow downstream. This stabilization of discharge is essential for both human water use and the survival of aquatic life in the downstream environment.
Sediment Trapping
The reduction in water velocity also causes sediment trapping. When river water enters the still lake environment, the suspended sediment load—such as silt and sand—settles onto the lakebed. This sedimentation results in the outflowing river having significantly clearer water than the inflowing river, as the lake acts as a natural filter. Over long periods, this process can build up deltas at the inlet and gradually reduce the lake’s storage capacity.
Thermal and Chemical Changes
The standing body of water alters the river’s thermal and chemical profiles. Deep lakes stratify during warmer months, forming distinct warm surface and cold deep layers. The water released into the outlet river will often reflect this lake temperature, which can be warmer or colder than the river would naturally be. The lake also influences dissolved oxygen and nutrient content. Decay of organic material in the cold bottom layer can deplete dissolved oxygen. If the outlet river draws from this deeper, oxygen-poor water, it introduces altered chemical conditions downstream, potentially including a release of nutrients like phosphorus.
Lakes That Do Not Connect to Rivers
While many lakes are integrated into river networks, certain types of water bodies exist outside of the typical through-flow system.
Endorheic Basins
One major category includes lakes in endorheic or closed basins, which are landlocked and have no surface outlet river leading to the ocean. In these systems, all water loss occurs through evaporation or seepage into the ground. These internal drainage basins are often found in arid or semi-arid regions where evaporation rates are high. Since minerals and salts carried in by tributary rivers cannot exit the basin, the water in endorheic lakes, such as the Great Salt Lake, accumulates high concentrations of dissolved solids, making them saline.
Oxbow Lakes
Other non-connected bodies of water are smaller, isolated features like oxbow lakes. These are crescent-shaped lakes formed when a river meander is cut off from the main channel after the river finds a straighter, shorter path. The oxbow lake remains separate from the active river flow and is only connected during extreme flood events.