Altai (Southern Siberia)
The Altai is the highest mountain range in Siberia, located in the south where Russia, Kazakhstan, China and Mongolia all come together. Plenty of good runs, most being multidays but by Siberian standards the area has a good road infrastructure so they are not too hard to reach.
When to go
The best season is July/August. It is possible to go slightly earlier or later but not ideal: September will be cold; late May/June is still in the high water season so it will be very wet and many runs will be too high. The weather is mild but unpredictable: many rivers are rain-fed so levels can be unreliable.
How to get there
There are plenty of flights into Novosibirsk, 600km north of the Altai range itself. A better option is to fly to Barnaul which is much closer but has less inbound flights.
The area has good infrastructure by Siberian standards, with some good quality roads. You will need your own transport rather than relying on buses/etc, 4x4 transport is recommended and mean you have to walkin less!