What’s up (or not) with Great Lakes water levels?

Some of the Great Lakes are expected to reach all-time low water levels this winter. Why do Great Lakes water levels change? We look at short-term lake level changes in the first of a two part series.

Part 2 of 2

Some Michigan residents keep track of the Great Lakes water levels by eyeballing them – keeping an eye on the lakes over time relative to coastal landmarks. Others keep track more analytically by looking at the Monthly Bulletin of Great Lakes Water Levels, produced by the U.S. Army Corps of Engineers. Whatever the case, Great Lakes water levels do change, whether over the course of a few days, a month, a year, or many years. The challenge is trying to predict some of those changes.Storm surge image.

Short-term changes are frequent on all the Great Lakes due to weather changes. If you know what storm surges and seiches are, you know about short-term water level changes. Storm surge is an abnormal rise of water generated by a storm, and it occurs where winds blow onshore. The National Oceanic and Atmospheric Administration (NOAA) has produced a video that shows a storm surge as it builds, demonstrating its potential effects on the coast.

There are several factors that contribute to the amount of surge a given storm produces at a given location:

1)  barometric pressure (lower pressures yield higher storm surges)

2)  storm intensity (stronger winds produce a higher surge)

3)  storm size (larger storms yield higher surges)

4)  forward speed of the storm (higher speed = higher surge)

5)  the storm’s angle of approach to the coast (storms that move onshore perpendicular to the coast generally produce a higher storm surge)

6)  shape of the coastline (storm surge will be higher when a storm makes landfall on a concave coastline [curved inward] as opposed to a convex coastline [curved outward]

7)  width and slope of the lake bottom (higher storm surges occur with wide, gently sloping lake bottoms, while lower storm surge occurs with narrow, steeply sloping offshore profiles)

8)  local features such as bays, inlets, rivers and islands

As for seiches, the word itself is French for ‘to sway back and forth’. Wind blowing across the water causes the water to move downwind, from one area of the lake to another. When the wind stops, the water sloshes back and forth (oscillates) until it reaches equilibrium once again. The sloshing back and forth is called a seiche. Seiches are standing waves with a central node and antinodes at either end of the lake.Siche in a lake image.

Among the Great Lakes, the most pronounced seiches occur in the lakes with an east-west orientation. Why? Because most storms in the Great Lakes move from west to east. Those lakes with an east-west orientation therefore present their entire length to the winds, and the longer the distance over which winds blow (this distance is called the fetch), the higher the waves and the greater storm surge. Why are seiches significant? They can cause water levels to change by several feet over a short period of time, resulting in damage to anchored ships, as well as coastal flooding and erosion. In addition to causing damage to shorelines and structures, storm surges and seiches may also impact the biology of lakes. If you’d like to “see” a 7’ Lake Erie seiche, check out a webpage developed by NOAA’s Great Lakes Environmental Research Laboratory that provides data as well as graphic and video imagery of the 2003 seiche.

For current information on winds, waves and water levels, visit the NOAA Great Lakes Coastal Forecasting System website. Additional information about Great Lakes conditions can be found on the Great Lakes Observing System website. And if you are an educator interested in teaching others about the effects of weather on the Great Lakes, check out the Teaching with Great Lakes Data website co-developed by Michigan State University Extension and Michigan Sea Grant.

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