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Majestic cottonwood trees, their shimering heart-shaped leaves blazing gold in the autumn sunlight, stand in regal splendor along the banks of the Rio Grande in New Mexico. A breeze rustles the leaves, and for a moment the cottonwoods, some more than 80 feet tall, seem to come alive, whispering secrets of the seasons gleaned from years of watching over the waters.

Known by its Spanish name, bosque, this venerable cottonwood forest stretches some 160 miles along the Rio Grande from Cochiti Lake in the north to just below Elephant Butte Reservoir. The bosque may have a timeless visage, but New Mexico’s native forest is dying: only mature and aging trees live there. Most of the cottonwoods are 40 to 80 years old, and natural regeneration is not replacing them. According to bosque restoration pioneer Cliff Crawford, “If current trees are not replaced by their offspring, the cottonwood bosque will be overwhelmed by non-native species and disappear within this century.”

The trouble is on the banks of the Rio Grande itself. Decades of taming the roughly 1,900-mile-long river have severely curtailed its movement. Where it once meandered over an ancient floodplain up to 4 miles across, the river is now constrained to a mere 1/4 mile in most areas, effectively eliminating the mosaic of cottonwoods and willows of varying ages that once grew there. In addition, the water flowing downstream from Cochiti Dam and other diversion dams has gouged and lowered the riverbed as far south as Elephant Butte Reservoir, depriving cottonwoods of their historic habitat―riverbanks low enough to be flooded with some regularity in late spring.

“Cottonwoods evolved with the river,” says Crawford, “so their 2- to 3-week germination window coincides with historical spring peak water flows of the Rio Grande.” Because of damming, channel-building, and urban development, the seasonal river fluctuations that once nurtured cottonwood seedlings no longer occur.

The other big problem is increasing competition from water-guzzling, non-native plants introduced in the early 20th century. Tamarisk (salt cedar), Russian olive, and Siberian elm are all taking a toll on the bosque.

More: Autumn Cottonwood Hikes