The Selous is closed from March till May because of the heavy rain season.
Between June and October all the animals will head to the river and the lakes. Then you’ll have hot days and cold nights. That’s a good time to discover the Reserve by 4x4 and I would recommend this time of the year.
In November till Februar the Rufiji River will have a lot of water, so that would be a good time for boat safaris as you will be able to explore more side arms.