Undergraduate oceanography

Understanding density stratification

Associated with chapter on Coasts, Beaches and Estuaries

Salinity gradients in estuaries

Evaluation of competing conceptual models

Working in groups

Here's the topic for the group to consider:

It is clear to us now that the mixing of freshwater and seawater in estuaries is dependent on many factors. You may be surprised to know that as late as the 1960s, many influential politicians and water-resource managers believed that estuaries were all vertically mixed (see the mixing diagrams in our text). In fact, jobs, scientific reputations and political power struggles resulted from the "heretical" idea in the early 1970s that San Francisco Bay can vary from a slightly stratified to a highly stratified estuary (a widely accepted property for the estuary during the Spring). When the estuary is highly stratified, long-term monitoring by the U.S. Geological Survey indicates that freshwater from the Sacramento and San Joaquin Rivers can actually be transported into the southern component of San Francisco Bay near the San Mateo Bridge.

If increased water diversions to Southern California from Northern California, via the California Aqueduct, were dependent on maintaining the proper bottom-water salinities in fish spawning grounds within the estuary, why does the form of estuarine mixing (i.e., vertical, stratified or salt-wedge) have such management and environmental significance? What are the potential problems for managers in assuming a vertically-mixed water column?

Let class groups work on this for about 20 minutes. Try to lead groups who seem to hit "a wall" or need clarification about the assignment.

Display an understanding of the implications of vertical salinity gradients developing under different estuarine-mixing regimes.