Silver Linings in a Seiche
Posted on: July 27, 2017

As Great Lakes basin residents, we know intuitively that water levels fluctuate year round. But are we able to identify why?

 

 

 

Three types of water level fluctuations occur on the Great Lakes: long-term (multi-year), seasonal (one-year), and short period (less than an hour to several days). According to Fisheries and Oceans Canada (The Canadian Hydrographic Service, Central and Arctic Region), a number of natural and man-made factors affect these levels.

Natural effects on water levels include:

  • precipitation on the lakes
  • run off from the drainage basin
  • evaporation from the lake surface
  • inflow from upstream lakes
  • outflow from downstream lakes

Man-made effects on water levels include:

  • diversions into or out of the basin
  • consumption of water
  • dredging of outlet channels
  • regulation of outflows

Most of us are fairly familiar with seasonal fluctuations, which see higher water levels due to spring and early summer runoff, and lower levels during the rest of the year. Many are becoming more interested in long-term fluctuations due to recent, headline-making extremes that some say are a sign of climate change. But how much do you know about the mysterious short term fluctuations? Today, we’re particularly interested in the seiche.

Seiche Story

Have you ever been on the shore of Lake Superior to see the tide roll abruptly out, only to come roaring back hours, or mere minutes, later? It’s likely you’ve experienced a seiche (“say-sh”). It occurs when wind or air pressure changes cause the surface of a lake to undulate rhythmically. The description is right there in the name – “seiche” is a French word whose meaning is “to sway back and forth.” The phrase was coined by Francois-Alphonse Forel, a Swiss scientist who founded limnology, the study of lakes.

A recent comment on InfoSuperior’s Facebook page from a Minnesota resident notes that Lake Superior’s seiche often produces much larger short-term water level swings than those which occur over a year or even a decade.

While many people explain the seiche as resulting from wind and waves pushing water to one side or area of the lake, thereby raising water levels (after which the water rolls back, or subsides, dramatically lowering water levels), air pressure is a much more important influence. The most dramatic Lake Superior seiches are often associated with poor, leaden grey or rainy weather when the lake is still and the wind is completely calm. Anyone living near the lake over a long period of time has witnessed this phenomenon. There are countless stories of boats floating gently beside a dock, only to have the seiche come in so that the walking surface of the dock is completely covered by water. Loosely tied boats float over the dock and when the seiche goes out, the boats come to rest on the dock – high and dry. These large-scale swings in lake level when it is flat calm are associated with high and low air pressure systems exerting varying levels of pressure on different areas of the lake.

Just as is the case with wind systems and the movement of air between high and low pressure systems, so go the waters of Lake Superior. Lake water is attracted, or rushes towards, low-pressure systems. The low air pressure exerted on the lake results in a “release,” or rising of lake waters under the low. The opposite is the case under high air pressure. The lake water constantly seeks equilibrium, even while air pressure systems exert varying degrees of force, often simultaneously, in various regions of the lake. This results in the seiche.

The Lake Is Moving

Strangely, most of the time we are not even aware of these water movements, even when a seiche may be occurring. The lake is moving though. Get close with the lake and you’ll notice. Head into a narrow channel between two islands, or a very narrow entrance to a bay. The movement of the seiche becomes very clear. The current, which is impossible to see in the open waters of Superior, once constrained through a narrow neck or channel, becomes like a river, full of current, eddies and even standing waves. For anyone using a motor, it is more difficult to observe these phenomenon. Using a paddle or oars in a kayak, canoe or rowboat, one becomes very aware of the seiche, as the current either assists, or makes paddling much more difficult.

Since these currents are associated with air pressure changes, they can also portend a change in weather. A general rule of thumb with Lake Superior seiches is that they will be most dramatic in extreme low pressure barometric conditions. Very extreme seiches may preceed, happen during, or after a low pressure system rolls through and often happen at all of these times, back and forth – rising and falling. Large scale seiches are often associated with instances of extremely heavy rain, counter-intuitively producing a large drop in lake level in a localized area, just when a huge volume of water is entering the lake. This, followed by a large water level rise. The casual observer simply sees water levels rising and falling, in an apparently bizarre manner, with no rhyme or reason. Be aware though, because a sizeable seiche in beautiful clear weather, can mean only one thing – a change in weather is coming and this change usually won’t be good.

Lake Superior regularly experiences small seiches that don’t make much of an impression on shore-dwellers. However, large seiches have the ability to cause major shoreline damage.  As can be seen below however, seiches can also benefit the lake. In fact the lake needs seiches. Small, shallow arms, baylets and inlets extending well inland depend on the seiche to provide hydration and life to all of the organisms that live there.

This homeowner video shows the rapid nature of the seiche.

Silver Linings?

While home owners and shipping industry employees may cringe at the thought of a strong, unpredictable seiche, an article by Ben Korgen of the Minnesota Sea Grant makes a case for its positive aspect. Lake biology may particularly benefit from a seiche. As Korgen explains,

Upper lake organisms eventually die. Bacteria decompose these dead organisms, converting them into nutrients or fertilizer for future generations. To support life, a lake needs mechanisms for lifting nutrients from deeper waters into the sunlit surface waters where algae use photosynthesis to create new living tissue.

While the occasional upswell and tidal current may lift nutrients in oceanic settings, they are inconsistent and undependable in Great Lakes ecosystems. Korgen’s article recounts how Minnesota Sea Grant researchers Steve Eisenreich and Joel Baker endeavoured to find out “how seiches might influence the biological economy of water bodies with low tidal ranges.” To do so, they started by studying the nepheloid layer, a “turbid, nutrient-loaded, particle-rich zone that hovers above the lake floor.”

After taking measurements of the nepheloid layer, Eisenreich and Baker found that seiches have one of two effects on the nepheloid layer: they maintain the layer, or they make it bigger. They observed organisms living in Lake Superior sediments stirring up nutrient-laden materials. These materials moved into the layer. Because the seiche increases the size of the layer, and generates currents within it, this material is moved for consumption by species living higher in the water column. Korgen summarizes:

“Seiches generate currents within the lake’s water column… A seiche-controlled nepheloid layer can be an important source of raw materials for the water column.”

Though it’s tempting to fear the seiche and its unpredictable effect on water levels, it’s an important reminder that Lake Superior’s ecosystems benefit greatly from them.


Further reading:

Fisheries & Oceans Canada: Canadian Hydrographic Service Central and Arctic Region: Fluctuations in Lake Levels – Types

Teaching Great Lakes Science – Surges and Seiches

Minnesota Sea Grant: Bonanza for Lake Superior: Seiches Do More Than Move Water

 

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