BOTTOM CURRENTS IN LAKE ERIE
               Robert P.  Hartley
          Chief, Surveillance Section
       U. S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
              GREAT LAKES REGION
           CLEVELAND PROGRAM OFFICE

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                             ABSTRACT







     In the summer of 1965, seabed drifters were released in Lake



Erie primarily along the south shore and in the western basin.  Re-



turns have indicated clockwise bottom eddies in the Toledo-Detroit



area, in the island area, and in mid-lake in the west half of the



central  basin.  They have also indicated eastward bottom flow in a



narrow band along the south shore and in a wider band along the north



shore of the central basin.  Centers of eddies may be significant



repositories for pollutants.  The center of the large clockwise gyre



in the central basin may accumulate material originating all along



the south shore of the central-basin.  The Toledo-Detroit eddy sug-



gests accumulation of materials therein from these cities.

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                             INTRODUCTI ON








      Many  attempts  have  been made  to describe the circulation  of  water



 in  Lake  Erie,  beginning  with the work of  Harrington  (1895).  Most of



 the work has  been on  surface currents and  has been concentrated  in  the



 western  basin.   Examples are the work of  Verber  (1955)  and Olson



 (1952).   In all  of  these studies,  either  floating cards and  bottles



,or  near-surface  drogues  were used.



      The Federal Water Pollution Control  Administration, in  its  pol-



 lution  investigations, recognized  the need for more  adequate lake flow



 data and conducted  a  current measurement  program throughout  Lake  Erie.



 The program was  similar  to that conducted in  Lake Michigan  by  Verber



 (1962),  using  Richardson self-recording current  meters.  To  supplement



 the Lake Erie program and to learn more about the transport  of bottom



 and near-bottom  suspended materials,  seabed drifters were utilized.



 To  our  knowledge this was the  first time  that bottom drifters  had been



 used successfully  in  Lake Erie. They have been  used successfully along



 the Atlantic  coast  (Bumpus, 1965)  and along the  Pacific coast  (Morse



 et  a I.,  1968).



                      METHODS  OF  INVESTIGATION



      The seabed  drifters (Figure  I) were  of the  type designed  by



 Woodhead and  Lee (I960), constructed of polyethylene plastic and  shaped



 like an  umbrella.   The disc was bright  yellow and the stem  was red,



 making  them easily  visible from a  considerable distance. The  stem  was



 weighted so that the  entire assembly  had  a slight negative  buoyancy In

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fresh water.  The tail drags along the bottom while the disc is pushed,



parachute-1 ike, by the near-bottom current.  The response to water



movement, according to the supplier, is \QQ% at speeds above 17 centi-



meters per second.  At 5 centimeters per second the rate of travel is



approximately 10% of true water speed.



     The drifters were marked with return-addressed adhesive-backed



labels which requested the finder to note the date and location found



and to return.  The labels were sequentially numbered with a laundry



marking pen.



     Ordinarily the enti% drifter was returned.  Most of the drifters



survived their travels in remarkably good condition, attesting to their



durability.  Nearly all were released during sampling cruises between the



first part of June and the middle of August, 1965.  Most releases



(Figure 2) were near the south shore of the central basin and around



the western basin because these are the areas of the most significant



tributary inputs to the  lake.  In addition a few drifters were dropped



in mid-lake and near the north shore of the central basin.  Two drifters



were dropped at each station.  In all, 368 drifters were released—



118 in the western basin, 234 in the central basin, and  16 in the



eastern basin.



                     RESULTS OF INVESTIGATION



     A gross summary of seabed drifter releases and returns is given in



Table I,  indicating the relative success of this kind of study in each



of the lake's basins and in the entire lake.



     Drifter returns began shortly after release and they were plotted

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                    TABLE I




SUMMARY OF SEABED DRIFTER RELEASES AND RETURNS
Place of Release
Western basin
Western basin
Central basin
Central basin
Eastern basin
Eastern basin
Tota 1 s
- Canada
- U. S.
- Canada
- U. S.
- Canada
- U. S.

Number
Releases
48
70
16
218
10
6
"368
Returns
U. S.
0
8
0
26
0
2
36
Canada
3
3
7
37
0
1
51
Total %
return
6.3
15.7
43.7
28.9
0.0
50.0
23.6

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according to shortest route.  The first returns (Figure 3) indicated


a dominant eastward nearshore flow along the south shore of the cen-


tral basin and a stronger eastward flow along the north shore.  Vel-


ocities are not known, but, if the shortest times are used, the flow


along the south shore netted I.I miles per day eastward, and along


the north shore 4.1 miles per day, also eastward.  Along the south


shore the drifters moving eastward averaged 12 times the distance of


those moving westward.


     In the western basin the first returns suggested bottom flow

                               5
eastward through the southern inland channels.  Along the Michigan


shore a flow westward toward shore was indicated off Toledo, while


near the mouth of the Detroit River a flow away from shore was sug-


gested.


     Returns during the first half of 1966 (Figure 4) correlated with


previous returns except that a westward bottom flow was  indicated at


the west end of the central basin.


     Drifter returns dwindled by the beginning of summer  1966 and the


project had apparently been completed.  Then,  in the fall of  1966,


nearly one and one-half years after the releases, a rash of drifters


from the north half of the  lake was returned  (Figure 5).  Some of the


drifters were found on shore, but most were picked up  in fishing nets.


From this time on nearly all returns were from nets and most were


forwarded to us by Dr. R. G. Ferguson of the Canada Department of Lands


and Forests.  Most drifters were found in the  broad bay between Pelee


Point and Pointe Aux Pins, Ontario, probably  because of relatively

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Intense commercial  fishing in that area.



     Returns were even greater in the spring of 1967 (Figure 6) and



were essentially from the same areas.  Another pause the following



summer was again followed by a similar pattern of  returns in the fall



(Figure 7).  For the first time a western basin drifter was also



found in the Pelee Point-Pointe Aux Pins area.



     It is significant that in the last one and one-half years only



two drifters have been found in the southern half  of the lake while



42 have been recovered from the northern half.  All but three of those



recovered from the north side were released near the south shore.



     Plotting all returns on the same chart (Figure 8) with near to



shortest paths presents a pattern of total confusion.  Many of the



paths in the central basin cross at high angles which is a most un-



likely occurrence in reality.



                            DISCUSSION



     Bottom drifters are similar to surface drifters in their  limita-



tions.  Only the end points of travel are known.  Travel paths and



velocities cannot be determined.  Minimum possible velocities can be



calculated but these have no great significance after the first few



returns.   It is  likely that  low current velocities can exist without



drifter movement and that occasional higher velocities can  indicate a



false net movement.  Wave action can also have the same effect.  The



type of bottom can affect velocities and number of returns.  For example,



one would expect many drifters to be caught in craggy rock surfaces, to



be slowed by dragging through  low-density mud, and to be most  responsive



over smooth hard sand bottoms.

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     Despite their limitations, it is assumed with some confidence



that the seabed drifters released in Lake Erie did respond fairly



well to important water motions.  With this assumption a movement



pattern has been developed on the basis of least conflicting lines



of flow, meaning simply that lines of flow should tend to be parallel



instead of crossing at high angles.



     The development of the bottom flow pattern (Figure 9) was vastly



aided by correlating the predominant directions of flow as recorded



by the  lowermost meters at current metering stations.  Although other



patterns have been drawn to fit the data and bottom topography, all



indicate similar major bottom flow features.



     In the western basin a «MM*VFclockwise bottom flow exists be-



tween the Detroit River inflow and the western shore of Lake Erie.



A predominant flow out of the northern channel, Pelee Passage, and a



lesser flow out South Passage are indicated with perhaps a clockwise



gyre around Pelee Island and Kelleys  Island.  Scarcity of returns from



releases in the mid-portion of the basin  indicate sluggishness of bot-



tom flow in that area.



     Bottom flow from the western basin appears to be directed south-



ward toward the south shore with some turning westward but most deflected



toward the east.  Some of this flow continues eastward as part of the



general eastward flow along the remainder of the south shore.  Much



appears to be caught up in a huge clockwise bottom flow gyre of the



central basin.  This gyre is centered about 20 miles south of Pointe



Aux Pins, Ontario in the western half of the central basin.  Flow  is

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strongly toward the east in the northern third of the basin while

the southern two-thirds is characterized by a slower westward flow.

The south shore eastward flow is a narrow band usually separate from

the gyre.

     In the eastern basin the bottom flow is less distinct but appears

to have a central clockwise gyre in the deep part and a strong eastward

south shore flow turning toward the north shore in the eastern half

of the basin.  There appears to.be a flow toward shore along most of

the north shore.  The south shore flow appears to feed the Niagara

River.

     The bottom flow pattern shown in Figure 9 Is caused primarily by

prevailing westerly winds and is consistent with them.  In general,

westerly and northerly flowing bottom currents are the balancing

response to relatively rapid eastward currents in the upper  layers.

Eastward flowing bottom currents are probably caused mainly by wind-

driven surface currents which reach to the bottom in addition to the

requirements for continuity of flow.

     The flow patterns of the central and eastern basins suggest also

a vertical circulation with upwelling along almost the entire north

shore and corresponding downwelling near the south shore.  This is

required to feed the eastward surface flow which is much greater than

could be fed by drainage inputs.

     In the western basin the bottom currents are probably dominated

by the large Detroit River inflow.  The  large flow out Pelee Passage
                                                                   •>
and the shoreward movement along the Michigan shore are reinforced .*

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by the prevailing winds.



                             CONCLUSIONS



     The seabed drifter project along with other investigations of



water motion in Lake Erie have led to several  conclusions important



to the studies of pollution.



     Suspended materials, many of which are damaging to the lake



environment, will accumulate in areas of eddy motion.  Such areas are



indicated off Toledo and near the Michigan shore, in the island area,



in the large eddy area in mid-lake in the western half of the central



basin, and in the deep part of the eastern basin.



     Prime sources of suspended materials which would be expected in



bottom flow are at or near the origin of the eddies.  For example,



wastes from Detroit and Toledo are fed into the eddy of that area and,



to a lesser extent, the one around the islands.  Wastes from the



entire south shore of the central basin, when they escape the nearshore



flow, feed into the large eddy In that basin.  It is suspected that



this becomes significant in late fall with colder tributary flow under-



running lake water and In spring with under-running of silt-laden trib-



utary water.  The paucity of mid-lake drifter returns in the fall of



1965 and spring of 1966 suggests that the drifters were mainly in the



nearshore flow zone until the spring of 1966, and then were forced lake-



ward by this phenomenon.



     Only the eddy in the eastern basin is remote from prime sources



of wastes, there being no large sources along the Canadian shore.



     Surface currents in the central  and eastern  basins originate along

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the north shore and move away from it toward the south shore.   This



surface water Is productive and as it nears the shore it becomes



more so.  This results in large crops of algae along the south shore.



When these algae die and sink they can be carried back to the  center



of the  lake by bottom currents where they damage bottom water  quality.



     The eddies in the western basin exist top to bottom, suggesting



that productivity should be higher in these areas.  The eddies in



the central and eastern basins are In bottom water only, Indicating



that productivity of surface waters would not necessarily be higher



in these areas.



     The lack of returns during the summer months from mid-lake in



the central basin indicates that a definite pattern of flow may not



have existed below the thermocline and that the drifters were  trapped



in the  hypolimnion.  Current meters have shown occasionally rapid



motion  in the hypoltmnion, but it is generally to and fro.  A  gyre-



like motion in the hypolimnion probably does not exist, while  above



the hypolimnion it does exist.  In effect then the thermocline, although



flexible, acts as the summer  lake bottom for the pattern of Figure 9.



     Bottom flow in the central basin is strong in  late fall and



spring  according to returns.   It is perhaps also strong  in winter but



there is little Chance of retrieving drifters during that time.  Many



Canadian fishermen report that drifters are most often found just after



a storm, particularly a northwester.  They also report that their nets



are cluttered with debris, organic detritus, and sometimes garbage.

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This suggests rather rapid transport from waste source areas and also



that direct Inputs of man-made wastes may have a measurable pollution



effect on mid-lake waters.  This has previously been considered In-



significant.



     One of the more important conclusions of this investigation is



that transboundary bottom water motion exists and It Is predominantly



from the American side toward the Canadian side.  If suspended pol-



lutlonal materials can be construed as similar In response- to bottom



drifters, we have strong evidence that materials originating on the



American side can be transported and deposited on the Canadian side



of Lake Erie.

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                            REFERENCES






Bumpus, D. F. 1965, Residual drift along the bottom of the continental



shelf in the Middle Atlantic Bight area; Limnology and Oceanography,



V.IO (Supplement): R50-R53.






Harrington, M. W. 1895, Surface currents of the Great Lakes as deduced



from the movements of bottle papers during the seasons of 1892, 1893,



and  1894; U. S. Dept. Agric., Weather Bur. Bull. B: p. IH4.






Morse, B., Gross, M. G. and C. A. Barnes  1968, Movement of seabed



drifters near the Columbia River: A.S.C.E. Jour. Waterways and Harbors



Div. V, 94, no. WWI: 93-103.






Olson, C. W.  1952, The Currents of western Lake Erie; Ohio State Univ.,



unpub. PhD dissertation.






Verber, J. L. 1955, Surface water movement in western Lake Erie; Proc.



International Assoc. of Theoretical and Applied Limnology, V.  12:



97-104.





Verber, J. L. 1962, Studies of currents in Lake Michigan and related



activities; Proc. 5th Conf. on Great Lakes Research, GLRD pub. no. 9,



p.  127.






Woodhead, P.M.J., and A. J. Lee  I960, A new instrument for measuring



residual currents near the seabed; ConseiI International Exploration



Mer. Hydrog. Comm., no. 12, Moscow, USSR.






Wright, Stlllman  1955, Limnological survey of western Lake Erie; Spec.



Sc. Report, Fisheries  139, U.S.F.W.S. 341 p.

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       List of  figures  for "Bottom Currents  in  Lake  Erie",  Hartley





Figure I  - Woodhead seabed drifter used in this investigation.


Figure 2 - Locations of seabed drifter release  in Lake Erie during  the spring


           and  summer of 1965.  Each circle denotes  the release location of


           2 drifters.   Releases totalled 368.


Figure 3 - Seabed drifter returns during last half of 1965.

                                                             •0

Figure 4 - Seabed drifter returns during first  half  of 1966.
               t

Figure 5 - Seabed drifter returns during last half of 1966.


Figure 6 - Seabed drifter returns during first  half  of 1967.


Figure 7 - Seabed drifter returns during last half of 1967.


Figure 8 - Seabed drifter returns .during all  of period from summer  1965


           through falI 1967.


Figure 9 - Prevailing bottom water circulation  as inferred from seabed drifter


           returns with paths plotted as near to parallel  lines and correlated


           with metered directions.

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YELLOW
POLYETHYLENE
                    RED
                    POLYETHYLENE
                 5 GRAM
                 BRASS WEIGHT
                                        55 cm
                                                        FIGJ

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