I love questions like this!
I did a search and found the page below in NASA's website:
http://visibleearth.nasa.gov/view_rec.php?id=5185
From this reference, it was "almost" completely frozen, so I wouldn't be surprised if it had, in fact, completely frozen over in recent years.
Great sheets of ice almost completely covered Lake Superior in mid-March of 2003. Winter clung tenaciously to North America, and was especially fierce toward the end, dumping record amounts of snow and clogging the Great Lakes’ shipping lanes with ice in a short period of time. It is a testament to the tenacity of the late winter’s strength that Superior froze over; the lake’s size and depth inhibit freezing, making this an unusual occurrence.
I found this resource on the tides:
http://140.90.121.76/faq2.html
Q: Are there tides in the Great Lakes?
The water levels of the Great Lakes have long term, annual, and short term variations. Long term variations depend on the precipitation and water storage over many years. Annual variations occur with the changing seasons. There is an annual high in the late spring and low in the winter. These changes occur at a rate which can be measured in feet per month.
True tides, changes in water level caused by the gravitational forces of the Sun and Moon, do occur in a semi-diurnal pattern on the Great Lakes. The investigations of the US Coast and Geodetic Survey indicate that the spring tide, the largest tides caused by the combined forces of the Sun and Moon, is less than 2 inches (5cm) in height. These minor level variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes. Consequently, the Great Lakes are considered to be essentially non-tidal.
Other factors may create quite large fluctuations of lake levels, but for only short periods of time lasting from minutes to several days. The two most dramatic are storm surge and seiche.
Hope that answers your question!
