An abrupt widening in a channel can act as a control on the regime of two-layer flow immediately upstream. A simple explanation in terms of an interfacial Froude number is offered and a set of experimental verifications is described. Bernoulli's Principle is applied to an abrupt narrowing.
Stommel, Henry, and Harlow G. Farmer. 1952. "Abrupt change in width in two-layer open channel flow." Journal of Marine Research 11, (2). https://elischolar.library.yale.edu/journal_of_marine_research/771