And the creatures above the pool: The damselfly alights on a reed and rests with its transparent, netlike wings closed above its slim body. The stouter-bodied dragonfly rests with its wings outstretched and likes to fly in tandem. Both of these aerial beauties must lay their eggs in water, and their larvae are fully aquatic predators that breathe with gills like fish. [Illustration: This flash flood (above) washed out a portion of the Maverick Road. Flash floods can be killers to the unwary. They can sweep down on you from storms you never saw or heard.] [Illustration: Cottonwood Creek’s wide bed suggests that it, too, knows rage. Low water levels favor algae growths whose colors mirror the cottonwoods’ refreshing verdure overhead.] Many of the same water insects inhabit yet another type of waterhole, the tinaja, a natural pothole that traps rain or runoff in solid rock. Dependent on rainfall, tinajas often dry up, yet they may be the only water source over a large area. If a tinaja is deep enough it may survive evaporation, but the water may shrink back so far below the lip of the bowl that animals cannot reach it. A cougar once drowned in a tinaja here because it could not climb out again. Tinajas may also turn into death traps for the plants and insects that inhabit them. In a well-balanced pool the algae create the oxygen and food that aquatic creatures need, but as the pool dries up there is less and less oxygen and the products of decay become concentrated. At last these become so poisonous that the reproductive engine cuts off and the pool is literally dead. But even a dead pool may be a source of life to outside animals. Ernst Tinaja is a good site for watching desert wildlife. It lies in a rocky, canyon-like drainage near the Old Ore Road. Though the upper tank measures 6 by 9 meters (20 by 30 feet) animals may not be able to use it because when the water is 3 meters (10 feet) deep it lies more than one-half meter (2 feet) below the edge. But mule deer and javelina frequent the smaller pools, which likely hold algae and a roster of aquatic life. Other important tinajas may be found on Mesa de Anguila. The mesa top has a maze of trails leading to and from tinajas that have served as a focus of life across countless centuries. You can find Indian shelters in the form of overhanging cliffs up and down a canyon, with a permanent tinaja right in the middle. Like the so-called lower animals mankind has long been dependent on waterholes. Since the first prehistoric Indians came to Big Bend, people have lived beside springs and tinajas. And what a pleasant prospect you still find from the sooted rock shelters above Croton Springs as you look out across the grasslands and the tules at the spring, toward the crenelated wall of the Chisos. Rounded red boulders beside the spring contain age-old mortar holes, ground so deep you can stick your arm in up to your elbow.