Does terrain matter, or is pokemon spawns completely random?

Around me, I find a lot of water pokemon around water.
I find some everywhere but when I get near water, the pop up like crazy.

Yes it does. For example water pokémons at the beach and pidgey's in a city. Last week on my vacation I found lots and lots of fighting Pokémon in 1 city, don't ask me why but I guess it ain't completely random.

thanks guys

Time of the day seems to matter around me too.