The houses on the Bayswater side of the river are often up quite a bit higher than the river itself. On the Maylands side you can see that the houses are up on a ridge, which gets quite high as you wind around. On the Bassendean side the houses are also up quite high with a lot of parkland in front. If you go down the the reserve there you will notice that you have to drive down a hill to get there. I would think there is minimal risk for even quite bad rains - given the norm for Perth is not as bad as other places.