Lakes partition themselves into temperature zones
Thermal stratification in lakes
•In deep lakes only the surface
layers are well mixed and quite
warm, whereas the deeper parts
remain cold.
•The thermocline occurs deeper
in large lakes because wind
energy is transmitted to greater
depths
 A0.5  2.35 
 
zt  ln 
 0.043  
zt = thermocline depth (m )
and A = lake Area (km2)
•Wind energy increases with
fetch
•In small lakes convection also
plays a role in determining
thermocline depth
Empirical formulae used to estimate wave height and wave length.
H max  0.332 F ,
where H max  max wave ht (m)
and F  max Fetch (km);
L  20 H
where L  wave length (m) and H  wave ht (m)
To estimate F, most people use either the max length of the lake, or the square root of lake
Area
The seasonal pattern of thermal
stratification in a deep temperate zone lake
Depth-time graph of isotherms
During spring turnover the entire
Water column is 4oC—why 4oC
Same thing happens again in the fall
Vertical thermal profiles
Heat diffuses much faster down the water column in large lakes—wind mixing
Hence the thermocline is
deeper in large lakes
depth of the thermocli ne
 A (km2 )  2.35 
 
Zt (m)  ln 
 0.043  


Fig. 12.7 in text
In small lakes mixing is more determined by convection currents driven by
solar heating and is determined by how deep light penetrates
Middle of thermocline
Top of the thermocline
In very large lakes horizontal
thermal shear zones occurs
at river mouths
A thermal bar
Important habitat feature for
many fish species in spring.
Waves- the gravitational response to wind disturbance
The bigger the
wind fetch the
bigger the wave
oscillation
The velocity in
these oscillations
attenuates sharply
with depth
Wave energy and
slope together
determine the
depositional zone
boundary
after Rasmussen and Rowan (1997)
Log DBD(m)=─ 0.107 + 0.742 Log F (km) + 0.0653 slope (%)
At depths > depostional
Boundary depth
fine mud accumulates