语法很厉害的过来帮我解答

anna100

See the following from Ron 大神：

“more than all the North American Great Lakes combined" is actually a MODIFIER of "20% of the world's fresh water", which is WITHIN the first MODIFIER. therefore, it's a SUB-modifier, so to speak.

let me try to illustrate it graphically:

More than 300 rivers drain into Siberia's Lake Baikal(, which holds 20 percent of the world's fresh water(, more than all the North American Great Lakes combined)).

the blue modifier modifies stuff that's inside the orange modifier, so it falls within the orbit of the orange modifier; it MUST be removed if the orange modifier is removed (because it has nothing left to modify). ”