levees, yes. if you're playing as the dutch, their dike UB replaces the levee, and it can be built on river cities and on coastal cities.
GMs, with him selected hold down shift and right-click on another civ's city. then hover over the trade mission button and that'll tell you how much you'd get if he was in the city this turn and could do it. it may change a little bit by the time you get there, if the city goes up or down in pop or builds a harbor or whatever but not drastically unless he whips half the pop away.
if you're playing warlords or BtS, often the city that has ToA gives far more than any other city, so i always check it even if it's small. generally, the further from your capital the better, and the more commerce the city has the better. which means more population and more trade-helping buildings in the city (harbor/custom house stuff) the better, so coastal capitals are often real good.