pokh More than 300 rivers drain into Siberia's Lake Baikal, which (=Siberia's Lake Baikal) holds 20 percent of the world's fresh water, (which (=Siberia's Lake Baikal) holds) more than all the North American Great Lakes combined. "More than all the North American Great Lakes combined" is the compliment of "which holds", so I wonder it doesn't modify anything. If I have to specify what is modified, I would say, it is the verb "holds".
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pokhMore than 300 rivers drain into Siberia's Lake Baikal, which (=Siberia's Lake Baikal) holds 20 percent of the world's fresh water, (which (=Siberia's Lake Baikal) holds) more than all the North American Great Lakes combined.