The sub-surface current patterns in South Fishtail Bay, Douglas Lake, Cheboygan County, Michigan

TitleThe sub-surface current patterns in South Fishtail Bay, Douglas Lake, Cheboygan County, Michigan
Publication TypeThesis
Year of Publication1967
AuthorsGannon JEdward
DegreeMaster of Science
Number of Pages52 pp.
UniversityUniversity of Michigan
CityAnn Arbor, MI
KeywordsWINDS
Abstract

Three sub-surface current patterns occur in the epilimnion of South Fishtail Bay - a clockwise eddy, a counter-clockwise eddy, and a convergent current pattern. The clockwise eddy, existing under northwest and north wind stresses, is the dominant current pattern in the bay based upon frequency and strength of these winds, length of fetch, and strength of currents produced. The convergent current pattern occurs under northeast and east wind stresses. This pattern is less important since these winds are much less frequent than northwest and north winds. Counter-clockwise eddy circulation occurs during southwest winds. These winds are very common but are offshore with respect to the bay. Therefore, counter-clockwise eddy circulation tends to be slower and more variable than the other circulation patterns. West winds are very common, but the bay is greatly protected from them. Currents under these winds are extremely variable and constitute a transition between the clockwise eddy of northwest winds and the counter-clockwise eddy of southwest winds. The importance of Grapevine Point Channel cannot be over-emphasized in relation to both forced, wind-generated circulation, and freely oscillating, seiche-induced movement in South Fishtail Bay. The channel acts as an escape mechanism for excess epilimnetic water during clockwise eddy circulation. It also acts as an escape mechanism after wind stress diminishes alleviating the pile-up water built-up during convergent current circulation. During counter-clockwise eddy circulation, inflow of water through the channel replaces water that has left the bay over Big Shoal. Movement in the channel is extremely variable during west winds. In the absence of wind stresses, seiche-induced circulation may be important in the constructed area of the channel. Drogues set in the thermocline tended to move generally in the same direction as epilimnetic circulation though at slower velocities. If these results are valid, then the method of energy transfer between the epilimnion and thermocline without altering density stratification is not known. There is a possibility that wind and/or surface current drag affected movement of the drogue systems in deep water. Perhaps the drogue systems used in this study are not sufficiently precise to accurately measure the slower water movements in the thermocline and/or hypolimnion. Movement in the hypolimnion was always slow and extremely variable. It is the only layer where no movement was occasionally observed. Due to this variability, the importance of seiche currents in hypolimnetic circulation has been suggested. A preliminary analysis of seiche activity revealed an average period of 8.00 hours with a standard deviation of +- 4.14 hours. More work is needed on circulation in the thermocline and hypolimnion, and on the importance of seiche currents in over-all circulation. With some knowledge of current patterns in South Fishtail Bay at hand, interesting work on the hydrodynamic aspects of fish movements, planktonic, and benthic distribution, and littoral flora and fauna distribution should be possible.