Quagga mussel (Dreissena bugensis) was discovered in Lake Mead, Lake Havasu and the Lake Mead fish hatchery in January 2007.  These are the first records for this invasive and potentially destructive species west of the 100^th meridian, which was previously known only in the Great Lakes and the St. Lawrence River.  Less is known about quagga mussel than about the similar zebra mussel (D. polymorpha).  Both species require at least 15 to 20 mg/L calcium to complete their life cycles.  It appears that rivers are successfully invaded only if an upstream lake or reservoir is invaded, due to the larval stage being planktonic for up to a month.  There is also evidence that these mussels rarely invade wadeable streams.  We use water chemistry data from the Enviromental Monitoring and Assessment Program (EMAP) to estimate the proportions of stream length in 12 continguous western USA states at risk of invasion by Dreissena spp.  We map hydrologic units (HUCs) with low, intermediate and high invasion risk, based stream calcium concentrations, from the EMAP data. SEQ CHAPTER \h \r 1