The centerpieces of the park are the two glacial "green" lakes. Why are they green? The answer is complicated. For one, the lakes are deep. Round Lake is measured at 180 ft deep, while Green Lake reaches 195 ft. Deep lakes tend to appear bluish because the wavelengths of light that can penetrate (and be dispersed at) great depths are those closer to the blue end of the spectrum.

Because of their depth and the high salinity of the basin waters, the lakes are meromictic and do not turn over and intermix waters like many other lakes in this region do. The Green Lake's cold and dense bottom waters tend to stay separate from the shallower, warmer waters. Because of this, sediment sinks and collects in the bottom and virtually doesn't decay. Since the sediment is not kicked up by mixing, the lakes do not take on a muddy, turbid appearance like other lakes do. Meromictic lakes also have still, mirror-like waters. The Green Lakes are no exception here. Their tranquil, reflective waters make for great photography.

The Lakes reside in an ancient river basin, carved deeper into the limestone bedrock by the last ice age. Limestone, an easily dissolved sedimentary rock, saturates the Lake's waters with calcium carbonate, a bluish salt solution.

Photosynthetic bacteria contributes to the geology (and to some degree, color) of the lakes by creating reefs of calcium/sulphur below the surface along the shore. You can see these structures, jutting out from the lake basin , just below the surface of the lake. They are most prominent near Dead Man's Point (see this Bird's eye photo). Look for sub-surface platforms that look like light-brown rock or sand, extending out from the lake shore and then dropping off suddenly.

Through a combination of their depth, high calcium carbonate concentrations and photosynthetic bacteria, the lakes maintain a bright aquamarine color. Hence the name.

These green lakes are just like those smaller waterfalls potholes, but on a massive scale. A retreating ice age glacier and an ancient river pouring from its melt water once existed here. This river created such a massive torrent of rushing water, that the plunge pool from a gigantic waterfall created the Green Lakes. The river dried up as the glacier disappeared and the lakes continued to dissolve away the limestone, expanding to their current form. Now the green lakes' basins are filled naturally with rain and ground water.

Much of Upstate New York, including the land that is now Green Lakes State Park was part of the Military Tract of the Revolutionary War: land that was surveyed and set aside as payment to servicemen for their participation in the war effort. Much of this land belonged to the Collin family, descendents of which still live adjacent to the park.

In the 1920s / early '30s Green Lakes was a popular tourist destination for the City of Syracuse and surrounding towns. The Green Lakes Landing stop on the Erie Canal, brought tourists to the northern end of the park by the boatload. Today, the land running along the Canal in Manlius is a park and walking/bike pathway. Follow the trail that leads from the small parking area off of Rt 290, across from the Green Lakes State Park north entrance. It will lead you to a footbridge over the Canal and to the Canalway Trail.

Green Lakes officially became a park in 1928 with the acquisition of 500 acres at the north end of the property. Additional acquisitions throughout the century expanded it beyond 2,100 acres. In 1975, Round Lake was designated a National Natural Landmark by the US Department of the Interior.

Pink Lakes
A layer of bacteria-rich water nearly a third of the way down into the lakes is pink rather than green. These reddish bacteria have found the perfect home at these depths and because the lake's waters do not turn over (mix) seasonally they stay at their optimal depths.

X-ray vision
It is difficult to see the bottom of the lake because of its mirror-like surface, but with a polarizer lens or polarizing sunglasses, you can cut through the glare and get a better view of organic material below.