Currents absolutely exist in lakes, representing the continuous movement of water within the basin. These movements are fundamental to distributing heat, oxygen, and nutrients, supporting the lake’s entire ecosystem. Lake currents differ fundamentally from ocean currents; they lack the predictable influence of tides and are significantly smaller in scale. Instead, their drivers are governed by local meteorological conditions and the unique physical properties of freshwater. Circulation can be complex, involving horizontal flow near the surface and density-driven movement in deeper waters.
Wind and Flow: The Primary Horizontal Drivers
The most common form of lake current is driven by wind stress acting directly on the water’s surface. Friction between the air and water drags the top layer along, creating drift currents. The strength and duration of the wind determine the speed and depth of this surface movement, which tends to be relatively shallow. This upper layer then transfers momentum downward by exerting a viscous drag on the water beneath it.
In large lakes, the Earth’s rotation, known as the Coriolis effect, influences these wind-driven flows. This force deflects the moving water to the right in the Northern Hemisphere, causing the surface current to move at an angle to the wind direction. This deflection results in complex circulation patterns, sometimes forming large, basin-scale gyres, or circular current systems.
Water flowing into and out of a lake also generates important directional currents, particularly near shorelines. Rivers entering a lake create a bulk flow, often called advection, that pushes water through the basin. This hydraulic gradient is noticeable at river mouths, where the inflow forms a distinct plume across the lake surface. Similarly, the outflow location creates a steady pull that contributes to the overall directional movement of the water body.
How Temperature Creates Internal Currents
Temperature variations create density differences that drive complex internal circulation patterns within a lake. During warmer months, many deep lakes develop thermal stratification, separating the water into three distinct layers. The warmer, less dense surface layer is the epilimnion, floating above the colder, denser bottom layer, the hypolimnion. The transition zone, where temperature changes rapidly with depth, is the metalimnion, which contains the thermocline.
This density layering acts as a barrier, preventing deep and surface waters from readily mixing. When a river enters a stratified lake, its water forms a density current by sinking to the depth that matches its own density. If the inflowing water is colder and denser, it plunges and travels along the bottom as an underflow. If the inflow density matches an intermediate layer, it spreads horizontally as an interflow, often traveling along the thermocline.
Wind action on a stratified lake can generate vertical currents near the shore, known as upwelling and downwelling. When wind pushes surface water toward one side, the water piles up and is forced downward (downwelling). Conversely, on the opposite side, surface water is pushed away, and colder, deeper water rises to replace it (upwelling). Upwelling often brings nutrient-rich, cooler water to the surface, altering local water temperature and chemistry.
Understanding Seiches and Other Lake Oscillations
Some of the most dramatic lake currents are caused by whole-basin oscillations known as seiches. A seiche is a standing wave that sloshes back and forth within a confined body of water, similar to water moving in a bathtub. These oscillations are initiated by a sudden force, such as a rapid change in atmospheric pressure or an intense wind event that pushes water to one end of the lake.
Once the initial force subsides, gravity attempts to restore the water surface to equilibrium, causing the water to rebound and oscillate until friction dampens the movement. The period of a seiche (the time for one complete cycle) depends on the lake’s length and depth, ranging from minutes in small ponds to several hours or days in the Great Lakes.
In stratified lakes, a similar but larger oscillation occurs internally along the thermocline, known as an internal seiche. Because the density difference between the epilimnion and hypolimnion is small, internal seiches can develop wave heights of 10 to 30 meters, even if the surface remains calm. These massive subsurface movements generate powerful, rhythmic currents that flow back and forth, causing significant mixing of heat and dissolved substances.