Oct. 26 Enroute to Ciudad Obregon and Return. The valley of the Rio Yaqui is a large alluvial fan, apparently old as the river itself is more or less confined to the northern edge of the valley, and there is but one main river bed, that of the Rio Muerte, but this is unimportant. A large irrigation ditch diverts most of the water from the river above Obregon and leads it into the rice fields, which occupy perhaps a third, or less than half of the entire valley. Along the road we travelled to Irrigacion, there were extensive tracts of uncleared, but varyingly grazed scrub consisting of several cacti and a number of thorn shrubs. This vegetation suggests that the valley flat was built up high enough to support a type which is probably climax, or at least fairly close to it. Probably diverting of the river water has had some effect on the valley flora, and the long-time grazing has also had some influence; but it does not seem that the changes have been such as to bring about an entirely secondary vegetation. The significance of this point is that the Rio Yaqui Valley does not now, nor has it probably in the recent past supported any subtropical vegetation such as occurs in valleys to the south.