RayRay, if it was flowing water since you called it a river then it wasn't a factor in the mosquito population since they require standing water.
As for your neighbors you can get pellets that are designed to kill larvae and just toss some in their pool during times of not being in use. Also you should contact your local Mosquito District Director and report that pool/neighbor as a nuisance area.
We often have notices sent out to remind citizens to empty stagnate water from buckets, old tires, birdbaths or anything else that can catch and hold water during mosquito season. We can levy fines for failure to comply although as of yet we have never actually done that.