Predecessor-successor logic in a project is based on the fact that most tasks flow in a logical order, either because work demands it--you cannot hang drywall until you have finished building the frame--you have resource constraints--you have only one front loader so task A needs to finish before you can move the resource to do task B--or you simply choose the sequence. So in the schedule, we establish relationships between the tasks so the schedule dates will move consistent with progress.
However, this rule, as with most rules, is not effective 100% of the time. The dependency in this case is time; the start of the effort is not primarily driven by its predecessors. So having a task predecessor may not be logical and, therefore, deviation from this general rule would be okay.
I do suspect that the start of this task has some task dependencies but you can create a date constraint in this case because it is logically date constrained. Since high and low flow of a river can be very accurately predicted--it is based on the month and the relative location of the moon and sun--you can establish a start date of this task based on the most likely date of low flow.
In the schedule, I would certainly load predecessors to those logical tasks that come before the construction to cross the river but I would hard code a start date of mm/dd/yyyy based on the most likely low flow date and use a no-earlier-than constraint. Therefore, the predecessor tasks will push out the construction start freely if your project is slipping but the construction start cannot start earlier than your predicted date.
Despite our ability to predict high and low tides quite accurately, there is always some variance. This is where risk management comes to play. If you chose, as an example, a start date of March 5, 2016, for this construction, you can have a documented opportunity that an early start between February 25 and March 4 is possible. You can exploit this opportunity by having your construction resources at the ready say starting February 28 in the event this opportunity comes true. Of course, of some of the predecessor tasks slips, then your opportunity disappears. Nevertheless, you were ready for it.