An Overview of Dangerous Currents on the Great Lakes
There are several dangerous currents that can get a swimmer in trouble on the Great Lakes. The currents that have most often claimed lives over the past 12 years (2002-2013) are structural currents, rip currents, outlet rip currents, and channel currents. According to the National Weather Service (NWS) Great Lakes Current Incident Database (GLCID), the Great Lakes averages around 12 drowning fatalities per year related to dangerous currents. Most of these dangerous current incidents occur on Lake Michigan because its eastern shores are positioned favorably for current development, and it is a popular tourist destination. Note that the incidents collected in the GLCID are current-related, meaning currents could have been only a partial cause of the fatality or rescue (likely a combination of high waves and currents).
When Great Lakes current-related incidents are analyzed, several commonalities emerge. First, a majority of the incidents occur during, or just after high wave action. Most of the wave periods during the incident were short, meaning there is less time in between each incoming wave (less time for swimmers to recover in between waves). This suggests that waves are just as much of a dangerous hazard as currents. Secondly, most of the incidents occur near shoreline structures, such as a breakwall or pier.
Figure 1. A graph depicting the beach features present at each incident. A majority of the incidents were related to shoreline structures, such as breakwalls or piers. These are denoted as structural currents. Classic rip currents occur in regions where complex sandbar structures are present. Outlet type rip currents occur most often near river mouths or power plant drainage areas. Channel currents occur in channels dug out between the shoreline and an offshore feature. Incidents in the red boxes represent the number of incidents that occurred on 1 day where at least 20 people had to be rescued. This gives the appearance incidents have been increasing over the years, when it has likely remained fairly steady overall. Data from the GLCID.
Current-Related Incidents By Lake
Lake Michigan has the most current related incidents of all the Great Lakes. This is because Lake Michigan (mainly the eastern shore) is favorably aligned for the weather and wave conditions conducive to current formation, and many beaches along the shoreline have sandbars and breakwalls. Additionally, Lake Michigan sees more tourism than the other Lakes.
Figure 2. Number of current-related incidents by Great Lake (2002-2013 GLCID)
Conditions that Lead to Dangerous Currents and High Wave Action
Dangerous currents develop at times when the winds (hence waves) are blowing in an onshore direction (towards the shore), and wave heights are moderate to high. This can be seen by looking at the data from the GLCID, where a majority of the time, waves were onshore or close to parallel to shore and wave heights were generally greater than 3 feet. High waves are conducive to currents because it 'piles up' the water near the beach, which causes the surface of the water to be uneven. Water can also pile up near a shoreline structure, like a breakwall, or near a river mouth. The water must return into the lake or the ocean, so it flows outward as a rip current, structural current, or outlet current.
Figure 3. Orientation of the onshore waves to the shore during current-related incidents. Waves moving onshore at a 60-90 degree angle will most often lead to classic rip currents, whereas waves moving onshore at a more oblique angle (especially 30-59 degrees) will produce primarily structural currents. Keep in mind, however, that 30-59 degree angle approaches can also lead to classic rip currents that develop more diagonally to shore than typical classic rip currents. Most of the incident on the Great Lakes are due to structural currents and classic rip currents, so it makes sense that most of the onshore winds are approaching at more oblique angles. Data from the GLCID (2013)
Figure 4. Most current related incidents occur during moderate-high wave action. Incidents decrease dramatically after 6 ft because most people do not swim in waves above their head. Data from the GLCID.
Current Related Incidents on the Great Lakes
Figure 5 below shows the wave/wind directions that commonly cause current-related incidents on the Lakes (red arrows). The prevailing westerlies across the United States lead to winds commonly out of the west, southwest, and northwest. This is why a majority of the incidents (seen as the darker red colors) are seen on the eastern, southern, and northern shores of each of the Lakes. These wind directions are often associated with the passage of a cold front (which causes winds to veer from southwest to northwest over a time period). With stronger winds out of these directions, waves can get high on the lakeshore, which can develop dangerous currents.
Figure 5. Current-related incidents (rescues and fatalities) across the Great Lakes. Red arrows indicate wind/wave directions that commonly cause dangerous current development. Data from the GLCID
Lake Michigan: Tourism + Favorable Conditions and Features for Current Development
Onshore directed winds/waves, shoreline structures, warm water and air temperatures, and popular tourist destinations all come together in late July-early August to bring higher numbers of incidents (rescues + drowning fatalities) on Lake Michigan. It is important to realize that although there are many incidents that occur at these beaches, these numbers must be kept relative to the number of people that visit the beaches each year. For example, Indiana Dunes has seen around 15 current related incidents (2002-2013), however over 2 million people visit the park EACH YEAR! Similarly, Holland State Park (Ottawa County) has seen more than 100 incidents over the years, however they see around 1.99 million visitors per year. These beaches are all safe, however beachgoers must remain alert to the changing weather/wave conditions at the beach, and heed the advice of park patrol and lifeguards. If waves start to get above 3 feet, dangerous currents are possible. Also, shoreline structures and river mouths are favorable locations for structural currents and outlet currents-so swimmers should stay away from them no matter what the conditions!
Figure 6. Map on the left depicts the most common location for current related incidents, with red arrows showing hazardous wind directions. Additionally, the black bars indicate the orientation of some of the breakwalls located at many beaches along the shoreline. These are not realistic in length-they were drawn large for easy visibility/demonstration. On the right, is a map taken from the Pure Michigan Tourism webpage, which shows the most advertised beaches on the Great Lakes. Millions of people visit these beaches on a yearly basis. Data from the GLCID