A stream found in an area too arid to have spawned such a ﬂow is called exotic, the stream’s headwaters being in some far moister region. Permanent desert lakes, such as Pyramid Lake northeast of Reno, Nevada, are formed either from permanent springs or from the ﬂow of an exotic stream whose source is in a nearby mountain range. Pyramid Lake is fed by waters from the Truckee River, ﬂowing down from the Sierra Nevada. Some of the world’s great rivers are exotic streams. The Nile is perhaps the most famous of these, while the Rio Grande is the grand exotic stream of the American arid zone, with a length of 1,885 miles, making it the twenty-fourth largest river in the world. Receiving its water from the melting snows of the Rocky Mountains in Colorado and New Mexico, it ﬂows through desert for much of its length, taking contributions from a few major tributaries, the Conchos, Pecos, and Chama Rivers, and rising with the runoff from summer thundershowers. The upper Rio Grande essentially terminates at El Paso, Texas, where it is entirely diverted from its channel for human use. It does not resume signiﬁcant ﬂow until its conﬂuence with the Rio Conchos 250 miles downstream.