DYNAMICS OF LAKES 



Internal waves and seiches of large 

 scale often play an important role in 

 the circulation of moderate to large 

 lakes. In the Great Lakes, it has re- 

 cently been demonstrated that in- 

 ternal waves dominate the flow 

 regime during summer in the central 

 portions of the lakes — i.e., away 

 from the shore zones. It is generally 

 assumed that the energy of these 

 large internal waves is degraded into 

 smaller-scale motions that produce 

 mixing. But there is a complete ab- 

 sence of information on how this 

 degradation takes place; as a result, 

 we don't know on what days to ex- 

 pect or not to expect "good" vertical 

 or horizontal mixing. 



Another completely obscure aspect 

 of internal waves is the mass trans- 

 port they cause. Individual particles 

 execute back-and-forth motions in 

 waves, often over a period close to 17 

 hours, but there is also a residual or 

 "transport" motion on top of the 

 wave-induced movements. The latter 

 determines the bulk motion of any 

 admixture to the lake, and next to 

 nothing is known about it (in con- 

 trast to actual, instantaneous current 

 velocities, which have been measured 

 frequently and in many places). In- 

 deed, lack of information on mass 

 transport in a flow regime dominated 

 by wave-line motions (particularly in- 

 ternal waves) may be said to be the 

 greatest single "gap" in knowledge 

 concerning circulation problems in 

 lakes, particularly in the Great Lakes. 



Currents — Persistent currents are 

 usually weak in lakes, including the 

 Great Lakes, with the possible excep- 

 tion of Lake Ontario, wherein the 

 Niagara River plume may perhaps be 

 classed a "current." Apart from this, 

 the possibility exists that long, slow 

 internal waves of the "Kelvin" type 

 produce fairly concentrated currents 

 with a lifetime of at least several 

 days. 



Recent work has indicated the ex- 

 istence of such quasi-permanent cur- 

 rents near the shores of some of the 

 Great Lakes, but the evidence is far 



from conclusive. Observed currents 

 at moored stations in the shore zone 

 of the Great Lakes show a greater 

 degree of persistence than in the cen- 

 tral portions of the lakes, but the 

 spatial and temporal current structure 

 is too complex to allow reliable gen- 

 eralizations at present. Indeed, one of 

 the main conclusions one may draw 

 from recent work on coastal currents 

 is that the details are too complex, 

 and an experimental technique aimed 

 at the determination of bulk mass 

 transport in the shore zones (some 

 appropriate tracer technique, for ex- 

 ample) should provide more useful 

 information than further direct cur- 

 rent measurements, requiring the de- 

 ployment of a large number of 

 meters. Another important point is 

 that current structure around the 

 shores of the Great Lakes is different 

 from place to place as well as from 

 season to season — yet we know little 

 about current or mass transport "cli- 

 matology" even though this is most 

 important in connection with the use 

 of the lakes by man. 



Some turbulent diffusion experi- 

 ments have been carried out in the 

 Great Lakes recently, simulating sew- 

 age outfall and warm effluent dis- 

 charges. The data are mainly relevant 

 to an initial phase of dilution (just 

 after leaving the discharge), and even 

 in this connection it is not certain 

 that the diffusive properties deter- 

 mined would be similar to those in 

 other locations, where the current 

 structure may be radically different. 

 On large-scale mixing, data are quite 

 scant, but what information there is 

 appears to show that any effluents 

 discharged in the shore zone tend to 

 remain there for several days (a 

 phenomenon termed "coastal entrap- 

 ment"). Indeed, it is not at all clear 

 what the physical mechanisms are 

 by which coastal waters mix with the 

 main body of the lake. 



There is little or no direct informa- 

 tion on the connection between cer- 

 tain conspicuous thermal features of 

 the lakes (upwellings, the "thermal 

 bar" during the warm-up period) 



and any current structures that may 

 be associated with them. However, 

 theory suggests that some strong cur- 

 rents may accompany marked ther- 

 mal features of this kind. It is also 

 obvious that a sudden appearance or 

 disappearance of upwelling along a 

 shore has an influence on the water 

 exchange between the shore zone 

 and main lake mass. (See Figure VIII- 

 12) Similarly, the fate of heated 

 effluent may be very different from 

 that of effluent with no thermal 

 effects, because warmer and lighter 

 water may "slide out" over the rest 

 of the lake and assume a flat lens-like 

 shape. Such phenomena are known 

 to occur in rivers and estuaries but 

 no detailed observations in lakes seem 

 to be available. 



Modeling and Instrumentation 



Mathematical modeling of circula- 

 tion and mixing in lakes (specifically 

 the Great Lakes, or at any rate lakes 

 large enough for the rotation of the 

 earth to be important in their dy- 

 namics) is in its infancy, but some 

 good first steps have been taken in 

 the past twenty years or so. Numeri- 

 cal modeling on the lines suggested 

 by atmospheric work should be com- 

 paratively easy (a two- or a three- 

 layer model should be adequate), the 

 main problem being to display the 

 multitude of results in an intelligible 

 form. It should be added, however, 

 that no mathematical modeling has so 

 far even been suggested for the main 

 variable of practical interest, the total 

 mass transport in the shore zone (due 

 to currents and wave-like motions). 



The instrumentation available for 

 experimental work in physical limnol- 

 ogy has not kept pace with modern 

 developments in other fields of sci- 

 ence. One agency reported that it 

 had a 40 percent useful return rate 

 from its own moored current meters 

 during the 1969 summer season — 

 a completely unacceptable situation 

 which is nevertheless quite universal. 

 Available current meters are not suf- 

 ficiently sensitive at low speeds; they 



255 



