A seiche is a phenomenon involving the complex, often unseen, oscillation of the entire mass of water within a basin. While the surface of a large lake or harbor may appear calm, the water beneath can be moving in a massive, slow-motion wave. These long-period events can go unnoticed initially but can suddenly manifest as significant shifts in water level along the shore. The intensity and period of this rocking depend on the unique shape and dimensions of the body of water in which it occurs.
Defining the Phenomenon
A seiche is technically described as a standing wave oscillating within a partially or fully enclosed body of water, such as a lake, reservoir, or bay. The water does not travel forward in a crest-and-trough motion like a typical surface wave; instead, it sloshes back and forth across the basin. This rocking motion involves the entire water body, similar to how water moves in a bathtub or a cup of coffee that has been abruptly stopped.
The standing wave pattern features points of maximum vertical water movement, known as antinodes, which occur at the ends of the basin where the water level rises and falls. Conversely, points along the middle are called nodes, where vertical movement is minimal, but the horizontal flow of water is strongest. This oscillation is governed by the basin’s natural frequency—the specific rate at which the water body will naturally oscillate once disturbed. When an external force matches this natural frequency, resonance occurs, significantly amplifying the sloshing motion.
The Mechanism of Formation
The initiation of a seiche requires an external force to temporarily displace the water from its equilibrium position, acting as the initial push. The most common meteorological triggers are strong, sustained winds that push water toward the downwind side of the basin. Rapid changes in atmospheric pressure, such as those accompanying a severe weather front or squall line, also create a differential force that presses down on one section of the water body more than another.
Once the initial forcing mechanism (like wind or pressure gradient) stops or moves away, gravity attempts to restore the water to a level surface, causing it to rebound to the opposite shore. This rebound overshoots the equilibrium point, initiating the sustained, rhythmic oscillation characteristic of a seiche. The period of this oscillation—the time it takes for the water to complete one full slosh from one end and back—is determined by the basin’s length, depth, and contour. The period of the fundamental wave in a basin is directly related to the basin’s length and the square root of its depth.
For a large lake like Lake Geneva, the primary longitudinal seiche period is approximately 73 minutes, reflecting its specific dimensions. Shallow, elongated bodies of water, such as Lake Erie, are particularly susceptible to large seiches because their orientation often aligns with prevailing winds, allowing for maximum water displacement. A long fetch and a shallow bottom allow the external forcing to efficiently transfer energy to the water, pushing the lake into its resonant frequency for prolonged oscillation.
Where Seiches Occur and Their Impact
Seiches are a global phenomenon, occurring in nearly any enclosed or semi-enclosed body of water, from small harbors and reservoirs to massive bodies like the Great Lakes and the Baltic Sea. In the Great Lakes, water level changes can be substantial. Recorded events on Lake Erie have demonstrated fluctuations of up to 8 feet (2.4 meters) between the high and low ends of the standing wave. A similar event on Lake Superior caused the water level to rise and fall by over three feet (one meter) within fifteen minutes.
The primary danger associated with seiches is the rapid, unexpected change in water level and the strong currents generated near the nodes. These sudden shifts can strand boats when the water retreats or inundate coastal areas and flood docks when the water surges. The strong horizontal water movement at the nodal points can create destructive currents that pose a risk to swimmers and complicate navigation for large vessels entering harbors. Because the period of a large seiche can last for several hours, these oscillations are sometimes mistaken for tidal activity, but they result from atmospheric or seismic disturbances rather than gravitational forces.