Without looking up a formal definition, how I believe the common person would differentiate between a river and lake are the following:
* Is the body of fluid, within a set of bounds, mostly still (and not flowing)?
* Is it closer to a:
* > bowl (hemisphere with cut side facing up)
* > sliced cylinder (again flat slice facing up)
* > sheet* (a large thin expanse)
* Is the fluid at the bottom shifting location, how quickly?
Rivers tend to have high flow to volume ratios, faster moving flows at depth, are usually more shallow, and are defined more like a squashed tube in natural states.
Lakes tend to be deeper, more stable (slower flow, if discernible at all), and are generally placid. Lakes /usually/ have a large dimension in at least two surface directions while rivers usually have that in only one.
Exceptions to the above occur with canals/channels* (though that's an ocean thing) which might be closer to the fuzzy boundary.
River deltas also occur in high sediment deposit areas, such as the ends of rivers where they transition in to lakes / oceans; the extreme end of a river delta being bogs and other swamp like areas with shallow slow moving water. (I argue that such areas are neither river nor lake, but a third category.)
Lakes tend to be deeper, more stable (slower flow, if discernible at all), and are generally placid. Lakes /usually/ have a large dimension in at least two surface directions while rivers usually have that in only one.
Exceptions to the above occur with canals/channels* (though that's an ocean thing) which might be closer to the fuzzy boundary.
River deltas also occur in high sediment deposit areas, such as the ends of rivers where they transition in to lakes / oceans; the extreme end of a river delta being bogs and other swamp like areas with shallow slow moving water. (I argue that such areas are neither river nor lake, but a third category.)