Lavender, a veteran chronicler of Western exploration and settlement, here narrows in on the Colorado River area--chiefly to recount, almost without dramatic or thematic shaping, the who-what-when of Spanish penetration, trapper reconnoitering, Mormon pioneering and, from the expeditions of John Wesley Powell, systematic surveying. The one-map, sparsely illustrated text, speckled with local place-names and references to specific sites, virtually presumes a knowledge of the terrain; the lack of historical signposts, along with the plethora of miner figures and incidents, calls for a prier acquaintance with the course of events. Two sequences, however, benefit especially from Lavender's careful research (the availability of a particular archive, he suggests, triggered the book) and his dispassionate weighing of facts: the messianic and entrepreneurial spread of Mormon settlement (the costs to the settlers, their equivocal relations with the Indians, the underlying Mormon/US clash); and just what river-traveler Powell did--as against what he later wrote he did. In the book's last quarter, it also acquires an implicit if not really articulated focus: a chapter entitled ""Exploitation Begins"" deals briefly with railroad surveying and mining claims; ""Selling the Scenery,"" on Indian monuments as well as the Canyon, includes the book's freshest material; ""Claiming the Water"" succeeds in explaining why Western water law is so complex. Within the region, an authoritative summary; elsewhere, useful as a spot-reference.