A Dynamic System

Prior to measurable human influence in the 14th century, the Rio Grande probably flowed year-round, as a meandering, sinuous, braided stream. That is, rather than being confined to a single channel as it is today, the location of the river moved within the floodplain. Most unregulated streams form a series of smooth, loop-like bends called meanders. These meanders occur in channels with low-gradient slopes because energy generated from flowing water spreads out as evenly as possible along its course. The location of the meander, and thus the river, shifts when swift currents erode the outer bank of a meander bend and slow water deposits sediment on the inner side of the meander. Over time, the river will shift and move considerably in this way. Sometimes an entire loop is cut off from the rest of the river as the river tries to take the shortest route, forming an oxbow. This area remains wet, like a small pond or marsh, for some time and then slowly dries out, resulting in changing plant communities. The river was also once more braided than it is now. A braided river consists of two or more interconnected channels separated by sand bars or islands. An active river may flow in only a portion of the braided channels at a given time, but during a single season may shift and move across most or all of the channel system. Occasionally the location of the river channel shifts abruptly on the broad floodplain as major floods cut new channels and drop sediment to block off old ones.

Because of the dynamic nature of the river, the Middle Rio Grande Valley would have continuously supported a patchwork of plant communities. Clearly all such communities, including the cottonwood forest, depend on the stream movement for their existence. Sand bars formed by the shifting river provide the conditions needed for cottonwood establishment. Seedlings that germinate on a newly scoured sand bar produce a cohort of plants of the same age, and if these survive future periods of high water, they will grow into a stand of young trees of abut the same size. New seedlings cannot grow in a forest with a closed canopy, which prevents adequate sunlight from reaching the floor, but as the river shifts its course, it may knock out a section of mature forest, thus providing the space needed for the establishment of new stands. The result is a variety of patches ranging from newly established seedlings to old, mature stands of huge trees to open areas with few trees. Such a patchwork of vegetation types is termed a mosaic. In addition to the more catastrophic effects of large floods, the bosque once benefited from the effects of annual inundation (the flood pulse) as the river overflowed its banks during the high spring runoff. Such low-intensity flooding provided water and nutrient-laden sediment to the forest. It also influenced a variety of processes, such as the rate at which nutrients cycle through the system, and limited the accumulation of leaf litter and woody debris on the forest floor.

Historically, these forest patches were interspersed with wetland areas. A wetland is a transitional zone between terrestrial (land) and aquatic (water) systems, where the water table is at or near the surface or where land is covered by water at least part of the year. Wetlands in the Middle Rio Grande Valley included wet meadows, marshes, sloughs, ponds and small lakes. In combination, these wetland areas were probably a significant component of the floodplain ecosystem, greatly affecting the vegetation and animals present. They were formed partly by the meandering nature of the river and partly by the high water table in the valley; in some areas the water table was actually at the surface, supporting water-loving plants. The resulting mosaic of vegetation types, consisting of patches of cottonwood forest of different ages mixed with various wetland communities and open areas of low terrestrial vegetation, in turn supported a large diversity of organisms. As a consequence, the Middle Rio Grande Valley had an extremely rich assortment of plant and animal life.