It is possible to hypothesize the stream was enchanted by
a few of the nazgul, on orders and directions from Sauron,
as part of a plan to isolate the elves (and collaterally the men
of Laketown) as part of a plan to prevent northwestern M-E collaboration
in defense. The stream to the west, giant spiders et. al. to the south,
Smaug to the east, and orcs in the mountains north of Mirkwood
would isolate the strongest single force in Rhovannion (the wood
elves of Thranduil). The stream's purpose could be seen not as an
absolute barrier but rather as akind of speed bump, together with
harassing evil creatures lurking around it. (All them eyes at night!
)