FW Eco Lecture 9 Lake Movements

What are the main causes of lateral water movement in lakes?

Tides, earthquakes, volcanic explosions, influx of water, Earth's rotation (Coriolis effect), and wind. Wind drives most movement through waves, Langmuir circulation, Ekman drift/spirals, and seiches.

How important are tides, earthquakes, and volcanic explosions in lakes?

Tides are measurable in very large lakes but are small (maximum about 20 cm in Lake Superior); earthquakes can cause major waves but are rare in lakes; volcanic explosions are generally unimportant.

When is influx of water especially important for lake movement, and what determines where incoming water flows?

Influx is especially important in reservoirs with short water residence times. The density of incoming river water relative to lake water determines whether it flows over, under, or between existing layers.

What are overflow, underflow, and interflow?

Overflow: incoming water is less dense than the epilimnetic water and flows near the surface. Underflow: incoming water is denser than the hypolimnetic water and sinks beneath it. Interflow: incoming water has an intermediate density due to temperature, salinity, or sediment and flows between layers.

What is the Coriolis effect, and how does it affect lake currents?

The apparent deflection of moving water caused by Earth's rotation. In the Northern Hemisphere, moving water is deflected to the right.

What is Ekman drift, and how does it differ from an Ekman spiral?

Ekman drift is the Coriolis deflection of wind-driven surface currents. An Ekman spiral occurs when surface water drags deeper layers and the current direction changes with depth because of the Coriolis effect.

What are progressive waves, and what determines their height?

Progressive waves are wind-generated surface-traveling waves. Wave height increases with wind strength and fetch.

What is fetch, and how does it affect lake mixing and stratification?

Fetch is the distance over water that wind blows without a major obstruction. Greater fetch can produce higher waves and deeper surface mixing, influencing thermocline depth and epilimnion thickness.

What is Langmuir circulation, and how does water move in this process?

Langmuir circulation is a wind-driven pattern in which water moves downwind in a helical or rolling motion, forming parallel circulation cells.

What causes the parallel streaks of foam, seaweed, or debris seen on lakes?

Langmuir circulation creates surface convergence zones where floating material accumulates, forming parallel streaks called windrows.

Why is Langmuir circulation important?

It is a major surface-mixing mechanism that can mix water several meters deep, depending on wind strength. It can increase thermocline depth, alter plankton distribution, and affect particle tracking, such as oil spills.

What is a seiche?

An oscillation of an entire water body or of water layers within it.

What is a surface seiche, and how does it form?

A surface seiche is an oscillation of the lake's water surface. Wind pushes water toward one end of the lake; when the forcing changes or stops, the water rocks back and forth under gravity.

What is the equation for the period of a surface seiche, and what do its variables mean?

T = 2L / √(gz). T = period, L = lake length (m), g = gravitational acceleration (m/s²), and z = mean lake depth (m).

How do lake length and mean depth affect the period of a surface seiche?

A longer lake generally has a longer period, while a deeper lake generally has a shorter period, according to T = 2L / √(gz).

How large are surface seiches, and what extreme example did the lecture give?

Surface seiche heights are usually small, often millimeters, but can be much larger. The 1954 Chicago seiche on Lake Michigan produced a wave greater than 10 feet.

What is an internal seiche, and how does it form?

An internal seiche is an oscillation of the boundary between water layers, especially the thermocline. Wind piles up the epilimnion at one end of a lake; when the wind stops, the thermocline rocks back and forth.

What condition is required for an internal seiche?

The lake must be thermally stratified, with water layers of different densities.

How does an internal seiche differ from a surface seiche?

A surface seiche involves oscillation of the lake's water surface; an internal seiche involves oscillation of water layers and the thermocline. Internal seiches generally have much greater heights and periods.

What is entrainment in a stratified lake?

The weakening of the thermocline that allows mixing between the epilimnion and hypolimnion.

How can internal seiches and entrainment affect nutrients, oxygen, and algal blooms?

Internal seiches can cause upwelling and mixing. Upwelling can bring oxygen to deeper water and nutrients toward the surface; entrainment allows mixing across the thermocline and can contribute to algal blooms.

How can you detect an internal seiche?

Use thermistors to measure water temperature at different depths and detect oscillations of the thermocline.

What are the major effects of seiches?

They can increase lake mixing, transport oxygen to depth and nutrients to the surface, contribute to algal blooms, create dangerous rogue waves near shore, and increase connectivity between bays and the central lake.

What is the overall role of wind in lake movements?

Wind drives progressive waves, Langmuir circulation, Ekman drift and spirals, and surface and internal seiches. These processes make wind a major driver of lake water movement.