Transport and Mixing Processes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 357

enough, the interface may temporarily contact the surface, creating an upwelling. Upwellings com-
monly result in locally colder surface water temperatures, in comparison with other surface waters.
Upwellings can also introduce additional nutrients to the surface waters from the colder and often
more nutrient-rich metalimnion. Oxygen concentrations may also be depressed with the introduc-
tion of hypoxic subsurface waters. Ford and Johnson (1986) observed that an upwelling following
a large storm depressed the surface dissolved oxygen concentrations of C.J. Brown Lake, Ohio, to
about 2 mg L -1 .

13.2.6 e artH ' S r otatIon —t He c orIoLIS f orce
The rotation of the earth also impacts water movement. Coriolis forces, due to the rotation of the
earth, cause currents in the Northern Hemisphere to be delected to the right, and to the left in the
Southern Hemisphere when an observer looks in the direction of the currents. The affect is typically
important for lakes, such as the Great Lakes. Jin et al. (2002) demonstrated the importance of the
Coriolis force for Lake Okeechobee, Florida.

13.2.7 p enetratIVe c onVectIon
Another surface-mixing phenomenon occurs as surface water cools during the nighttime or during
seasonal cooling periods. The cooler, denser water overlying the less dense, warmer water causes
negative buoyancy or unstable stratiication. This density instability causes the cooler surface
waters to sink and the underlying warmer waters to rise, mixing the upper layers. This mixing will
continue until a stable density proile is once again achieved.

13.2.8 I nfLowS and c urrentS
The impact of inlows may be illustrated by longitudinal variations in reservoirs, as discussed in
Chapter 12 and illustrated in Figure 13.9. In the riverine zone, the initial momentum of an inlow
into a lake or a reservoir pushes the more stagnant lake water ahead of it until the initial momentum
is substantially dissipated (Ford and Johnson 1986; Cassidy 1989). Prior to dissipation in this zone,
the advective riverine processes dominate the transport within a lake. In the riverine zone, turbulent
kinetic energy is usually suficient to keep the water column well mixed, preventing vertical stratii-
cation and the settling of some materials. The mixing between the riverine inlow and the reservoir
waters is limited and the quality or identity of the inlow remains similar to that of the riverine water
(Martin and McCutcheon 1999).

Dam

Riverine Lacustrine

Transition

River inflow

FIGURE 13.9 Longitudinal zones of a reservoir. (From Thornton, K.W., et al., Proceedings of the Symposium
on Surface Water Impoundments , Volume 1, American Society of Civil Engineers, New York, 1981; Wetzel,
R.G., Limnology , 3rd ed., Academic Press, San Diego, CA, 2001. With permission.)