WATER TRANSPORT STUDIES IN THE STRAITS OF MACKINAC REGION OF LAKE I-IURONI Charles I;. Powers and John C. Ayers Department of Zoology and Great Lakes Research Institute The University of Micliigan, Ann Arbor, Michigan ABSTRACT The Straits of Mackinac comprise five simple straits in the region embracing Mackinac Island, Round Island, Bois Blanc Island, and the adjacent portions of the Upper and Lower Peninsulas of Michigan. Were the inflows from Lakes Michigan and Superior enter Lake Huron. Direct assessments of the inflow from Lake Michigan and of the run-off from the Lake Huron watershed, hitherto obtainable only by indirect means, have been made. The stildy indicates that large volumes of water move horizontally in oscillatory motion, due to the action of the Lake Michigan-Lake Huron uninodal scichc, but that this motion has no net effect upon the flow-through of waters from the upper lakes through Lake Huron. The excess of oscillatory transport over net flow-through indicates the seiche has an amplitude of about 0.1 m, a value which is independently confirmed by data from the literature. INTRODUCTION The authors extend their thanks to Drs. George H. Lauff and E. B. Henson for as- The Straits of Mackinac present the physi- sistance in the synoptic survey, and to the cal limnologist with a challenging complex University of Michigan Biological Station of phenomena. A simple strait can be a for headquarters and laboratory facilities puzzling problem; the Straits present a mul- during the field phases of the studies. The tiple condition in which component simple studies were supported by National Science straits lie between the Upper and Lower Foundation Research Grant No. G-1256. Peninsulas of Michigan, between the Upper Peninsula and Mackinac Island, between METHODS Mackinac Island and Round Island, bc- tween Round Island and Bois Blanc Island, The essential methods were the synoptic and bctwecn Bois Blanc Island and the survey followed by computations of volume Lower Peninsula (Fig. 1). Situated at the transport by the dynamic height method, junction of Lake Michigan and Lake Huron, with subsequent checking of current veloci- the Straits serve as an outlet for Lake Michi- ties by submerged current drogucs. gan, but their function in this capacity is Dynamic height computations of volume complicated by the winds and seiches of transport through the two transects adjoin- the two lakes. Just east of the Straits, Lake ing Bois Blanc Island, in Mississagi Strait, Huron receives the inflow from Lake Su- and in the Manitoulin-Hammond Bay tran- perior through the St. Marys River, The sect were made by the method described by complexity of the region makes it a difficult Ayers (1956) and Ayers and Bachmann one to study, but its hydrographic impor- ( 1957). All these transects involved shoal tance as the origin of the major flow through water at their ends and the computations Lake Huron renders it of real interest, Our were carried out twice, being independently original purpose in these studies was to test computed using the Helland-Hansen ( 1934) the dynamic height method of volume- and Groen (1948) techniques for dynamic transport determination in a complex area; height computations in shallow water. that other and probably more important rc- On 17 August 1956, after predominantly sults could be obtained became evident as westerly winds of ten days duration, two the studies went on. vessels undertook a synoptic survey of the roughly polygonal area designated “Study 1 Contribution No. 6 from the Great Lakes Rc- search Institute, the University of Michigan, Ann Area” in Figure 1. One vessel made two Arbor. transects at the level of the eastern end of 81 82 CHARLES F. I’OWtiRS AND JOHN C. AYERS FIG. 1. Chart showing the “Study Area” and surrounding regions. Bois Blanc Island. The second covcrcd De- quarters through its section from Manitoulin tour Passage, False Detour Passage, Missis- to Hammond Bay when this wind began. sagi Strait, and then ran a long transect from Volume transports in False Detour Pass- Manitoulin Island to Hammond Bay on the age were determined from current meter Lower Peninsula. readings at each meter of depth. It had been The first vessel completed its transects, planned to make similar current meter R and C, sufficiently early in the day for measurements in Detour Passage and in its results to bc free of the effects of a Mississagi Strait, but heavy steamship traffic southeast wind which appeared in the late in Detour and excessive depths in Mississagi afternoon, The second boat was about three- made the necessary mid-channel anchoring impossible. Volume transport through De- TA~LI;: 1. Volume trunsgorts into and out of the tour Passage is based on the US. Lake Sur- Study Area, cubic feet per second vey’s gaugings of the St. Marys River at Sault Ste. Marie, Michigan. This figure and Hellancl- Groen Hansen Mean the outflows of the St. Clair River at the - _- -- - -- - - -.--- - - foot of Lake Huron used later were kindly In: provided by Mr. W. T. Laidley of the U.S. by dynamic compu- tation Lake Survey ( personal communication ) . Transects B and C 508,000 456,000 482,000 Mississagi Strait 279,000 230,000 255,000 RESULTS by gauging The volume transport figures obtained Detour Passage - - 72,000 by current meter from all methods are given in Table 1. “In” F&c Detour Passage - - 171,000 an d “out” refer to net transports into and out of the study area. Total in 980,000 We do not pretend that steady-state con- Out: by dynamic compu- ditions prevailed during the entirety of our tation synoptic coverage of the study area. The Transect A 655,000 812,000 734,000 southeast wind of the afternoon of 17 Aug- ust reached Beaufort force 3 and westward Excess in 246,000 currents were found in the southern end of WATER TRANSPORT IN LAKE HURON 83 transect A which was being covered at the from simultaneous bathythermograph data. time of this wind. Ayers et nl. ( 1956, Figs, Drogue velocity at 4.6 m was 10.0 cm/set 18, 33, and 48) indicate that currents along . and the computed velocity was 11.9 cm/set, the Hammond Bay shore are castward un- both south&t. At 13 m the drogue velocity der winds from westerly quarters. was 11.5 cm/set while that computed by There is every reason to believe, however, dynamics was 1.1 cm/set; no explanation that the volume transports obtained for can be given for the latter failure, but it was transects B and C, Detour Passage, False the only failure. All drogue studies were Detour Passage, and Mississagi Strait were corrected by the method of Volkman et al. free of effects of the southeast wind by vir- (1956). tue of being taken hours prior to the onset The total complex of data indicates that of the wind, We therefore consider that a our synoptic coverage of the study area, balanced condition obtaining before the prior to the rise of the southeast wind, was rise of the wind can be represented by the under a temporary balanced-state condition condition in which transports into and out in which transports into and out of the study of the study area are equal and that the area were equal, but in which a large volume mean value of inflow, 980,000 cfs, probably of water was moving in seiche motion that best represents the balanced-state outflow would be expected to be oscillatory and to through transect A. contribute nothing to the net downlake A volume transport figure of nearly a movement. Such a concept of a large vol- million cfs in the head of Lake Huron at ume of water in oscillatory motion in the a time when the outflow through the St. area is not unrealistic. Computations of Clair River at its foot was 198,000 cfs could the Lake Michigan-Lake Huron seiches by indicate either that volume transports dc- Ayers and Ayers ( unpublished ) indicate termincd by the dynamic height method that the Straits region contains a node of were drastically in error, or that steady-state each seiche of the bi-lake system, Water conditions in the ordinary sense did not ap- movements at a node are horizontal. Lo- ply at the time of our survey. That the latter cation of a node in this region is borne out was the case and that dynamic height com- by the common knowledge that the Straits putations are fairly accurate in the Straits region is not one of common or extreme region is indicated by the good agreement water-level changes, and that it is an area between surface velocity by dynamics and of strong and reversing currents. by drift bottles obtained by Ayers et nl. The volumes of southward transport ( 1956, pp. 28 and 60) at their 90-series sta- through False Detour and Mississagi Strait tions in this region, Volume transport by can be considered a normal result of the dynamic computation in False Detour Pass- eastward passage of the seichc with subsc- age on the day of the survey gave order-of- quent development of height diffcrcnce bc- magnitude agreement with that obtained by twecn North Channel and open Lake Hur- current meter ( 93,000 to 171,000). On 25 on. The velocities of flow computed for July 1957, after a westerly wind regime these passages were 36 and 28 cm/set re- comparable to that prior to the synoptic spectively, averaging 32 cm/set. This aver- study, two current drogues set at the loca- age can be substituted into the formula for tion of transect C gave 8.6 cm/set east at the speed of efflux of fluid from an opening, 1 m and 1.1 cm/set east at 50 m; these compare with 7.9 cm/set east computed by v = -L/ 2gh dynamics at 1 m and 1.0 cm/see computed and the hydraulic head, 0.5 cm, needed to by dynamics at 50 m, computations being produce this velocity, computed, North based on data from the synoptic study, Channel is almost completely cut off from On 6 August 1956, after westerly winds, the St.