Susan,

If you can pickup or request USGS Fact Sheet 034-02 (2002)

Mount Rainier--Learning to Live with Volcanic Risk

It has a nice discussion and map of potential lahar/mudflow/lava flow/flooding models.

Essentially the Elbe, Carbon, Puyallup, White and Cowlitz river valleys are at risk.

However this brochure and the presenation noted that lahar risk from Rainier is great due to its proximity to communities and all models are models only, and these volcanoes are not predictable.....preplaning is everything.