What type of reproductive isolation has occurred when a barrier, such as a river, separates two populations?

the founder effect is when some individuals of a population are forced away from the original population, eg by a rise in sea level creating an island, so there is a smaller range of alleles and the new population is much less diverse.What type of reproductive isolation has occurred when a barrier, such as a river, separates two populations?
Allopatric Speciation. This is when the two separated populations can no longer mate due to the physical barrier.What type of reproductive isolation has occurred when a barrier, such as a river, separates two populations?
Allopatric speciation, genetic drift