A. to impersonate a river one would have to say, run at different speeds, wind in and out around obstacles and jump down off say stairs to mimic a river that has a variable flow rate depending on its' grade, and running fast and straight would be like a young river which cuts through its obstacles (has a lot of energy) and run between two vertical walls to mimic a young river cutting its' own canyon. Running slowly around obstacles would mimic a mature to senile river which meanders to avoid obstacles and has less energy and meanders a lot. Jumping down off stairs could mimic a waterfall.
B.A river also forms an ecosystem for various forms of life like an environment for fish to live in like local resident fish like trout for example but also for salmon returning up the rivers to spawn and bury their eggs in to reproduce future salmon runs. In the tropics I believe that large snakes can live in rivers like say the anaconda and also I believe crocodiles can live there too.
C. The problems faced by a river while flowing are mentioned above that a river can make a waterfall where there is a cliff or scarp and as if flows it can erode the underlying rocks if stratified sedimentary rocks like at Niagara Falls and so that eventually the falls in eons of time will be eliminated depending on the stratigraphy and inclination of the sedimentary layers. Also, where rivers flow through the mountains like the Fraser River in British Columbia, Canada it flows through the Coast Ranges to get to the Pacific Ocean and in the course of doing so, cuts through the rock and through downcutting has formed a major canyon which is a sign of a young river. Further down the river between Hope and Vancouver, there are meanders in the wide valley so this is where it has a lower grade and less energy and so goes around obstacles so can be relatively young in one part and mature in the lower part.
