• Vertical density differences are a common feature of lakes and estuaries, and sometimes of rivers.

• Density gradients are caused by differences in water temperature and in dissolved solids (TDS) or suspended solids (TSS).

• In freshwater lakes, temperature is the cause of density stratification.

• In estuaries, salinity is the primary cause of density stratification.

• In rivers, suspended material may influence stratification.

• Stratification in a lake may be weak or strong.

• The Richardson number describes the stability of a stratified condition in a lake:

  Ri = - (g/ρ) (dρ/dz) / (du/dz)2 ρ= density z= vertical distance u= current velocity g= gravitational acceleration

• When R_i = 0, the stratification is neutral.

• When R_i > 0, the stratification is stable.

• When R_i < 0, the stratification is unstable.

• The incoming water volume to a lake may flow over, under, or through the main water mass as a result of gravity and density, leading to density currents.

• If the density gradient causing the density current is due to suspended material, the phenomenon is called a turbidity current.

• Turbidity currents are common in estuaries and large reservoirs.

• Factors determining lake temperature are latitude, altitude, and continental location.

• Within the midlatitudes, lake temperature is strongly variable during most of the year.

• The sun is the principal source of energy.

• The visible-range wavelengths are almost totally absorbed within the upper 2 m.

• Wind is the distributing force for the heated water within the basin.

• Water's high specific heat is another important thermal property in lakes.

• This results in large storage of thermal energy and slower rate of heating and cooling than the surrounding land surface.

3.8  THERMAL CYCLE IN A TYPICAL MIDLATITUDE LAKE

• Most lakes are located within midlatitudes and undergo seasonal temperature fluctuations during the year.

• Seasonal variations of temperature lead to three well defined layers developed during the summer:
  
  
  • The upper homogeneous warm layer, or epilimnion.

  • The lower fairly homogeneous cold layer, or hypolimnion.

  • The intermediate layer characterized by a sharp temperature gradient, or thermocline.

• Epilimnetic water gradually heats until the latter part of July.

• The thermocline depth increases throughout the summer until it finally breaks up completely in the fall.

• The initiation of fall circulation (complete mixing) is a direct function of the frequency and force of winds and the seasonal decrease in temperature.

• As temperature decreases further during total circulation reaching 4^oC, and during a calm, cold night, typically in December, an ice cover will usually occur.

• The ice cover will continue to increase during winter.

• Temperature in the hypolimnion gradually increases through the summer.

• Turbulent transfer of heat and upwelling may be responsible for this process.