I have heard this expression used to describe a situation that has considerable concern about the outcome, as in a flood of uncertainty. An example would be, “The way the stock market is acting lately creates a river of doubt.” I was unaware that the expression originated with the exploration of the Rio da Duvida or the River of Doubt in Brazil. The book “The River of Doubt, Theodore Roosevelt’s Darkest Journey” describes the exploration and the brutal conditions the expedition faced. There is a review of that book posted on that link of this web site.