Lake Michigan environmental survey : final report / [edited] by John C. Ayers.

carried out by the then U. S. Public Health Service.
The USPHS worked from river flows of 1963-64 and average concentrations
of solutes to arrive at each river's loadings of solutes that were supplied to
the lake in 1963-64. Their results are given in Tables C-1 through C-6. The
only thing we have done is to sum in these tables the pounds per day of loadings delivered to the lake from these 19 tributaries in 1963-64, These sums
are presented in Table C-7. The inputs to the lake in Table C-7 are given as
actual sums of the USPHS findings and as accepted values, which are the sums
rounded to the nearest 1,000 lb. per day. They are conservatively low because these tributaries do not drain all of the watershed which discharges into
Lake Michigan, and comparisons of total long-term mean flows of 15 of these 19
tributaries to the total 1963-64 mean flows of the same 15 as recorded by
USPHS indicate that the flows of 1963-64 were about 19999/25501 of long-term
mean flows.
Of the 41,041 square miles of the Lake Michigan watershed only 31,940
square miles have gaged drainage. We have assumed that the inputs summarized
in Table C-7 represent practically all the gaged drainage, and multiplied by
41041/31940 to correct for ungaged drainage. We have further multiplied by
25501/19999 to correct the 1963-64 flows of Table C-1 to the long-term flow.
The resulting estimates of total inputs into the lake are given in the first
column of Table C-8.
The same FWPCA publication already referenced also gives in its Tables
10 and 12 analyses of water of Lake Michigan adjacent to Grand Traverse Bay
and at Chicago. Figure C-1 presents our present best idea of the "normal"
(prevailing wind) circulation of Lake Michigan. In this figure is summarized
our evidence that water from off the mouth of Grand Traverse Bay departs quite
directly to the Straits of Mackinac and hence to outflow to Lake Huron. To use
the analyses of water adjacent to Grand Traverse Bay as measures of the water
outflowing into Lake Huron appears to be justified. These analyses, together
with mean outflow through the Straits of Mackinac, and the withdrawal of water
at Chicago permit estimates of the amounts of some solutes being removed from
the lake.
The USPHS1 derived the mean outflow through the Straits of Mackinac as
1500 m3/sec (53,000 cfs) and water is diverted at Chicago at a mean rate of
1. Lake Michigan Basin, Lake Currents. FWPCA, Great Lakes Region,
Chicago, Illinois. November 1967.

C-2