The Upper Watershed
The term watershed is often used when discussing river systems. The meaning of the term has changed somewhat from its original definition, particularly in the U.S. Originally, the term "watershed" referred to the boundary between catchments or drainage basins. That is, watershed referred to an area or ridge of land that separates waters flowing into different rivers, basins, or seas. Over the years, US scientists used the term watershed interchangeably with "drainage basin," so that now another accepted definition for watershed is an area or region drained by a river, river system, or other body of water; this is also the definition of drainage basin. We will use this later definition, since this permeates the literature that teachers are likely to encounter. A watershed can be viewed at many different scales. A small arroyo has a watershed, which is the area that drains water into that arroyo, while this same area will also be part of a large watershed that includes all the area that feeds the stream into which the arroyo drains. In this way, the watershed of a large river may contain thousands of smaller watersheds. It is important to remember that the watershed of a large river, such as the Rio Grande, extends far inland and up into the mountains. In fact, all land on Earth is part of some watershed, since water from any point of land eventually drains into a river system or a sea (or in some cases into a closed basin with no outflow). The watershed for the Rio Grande includes the three US states that the river crosses (Colorado, New Mexico and Texas), while in Mexico, the watershed includes five states (Chihuahua, Coahuila, Nuevo Leon, Tamaulipas and Durango) even though the river only passes through four of these states (all but Durango). Thus people, as well as plants and non-human animals, over a vast area are connected by this common watershed that supports the Rio Grande. We do not have to live immediately next to the river to affect it - actions taken far inland can have significant impacts as well.
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Most of our discussion in this guide focuses on the main stem of the Rio Grande in central New Mexico, which is a floodplain river. The upper reaches of the Rio Grande and its tributaries (all of which we will call "upper reaches" here for simplicity) differ in many ways from the Middle Rio Grande, both in the physical characteristics of the streams and in the species of plants and animals present. Because upper reaches each collect water from much smaller areas than the Middle Rio Grande, they contain much less water and thus are narrower. They tend naturally to be more contained within a narrow channel, and thus fairly straight, rather than meandering across a floodplain as does the lower river. Although smaller in size than floodplain rivers, these upper-reach streams tend to have faster flowing water, with alternating riffle (rapids) and pool sections. The streambed tends to include boulders and coarse cobble, rather than the finer gravel, sand or clay substrates seen in larger rivers. The water is generally cold and with few suspended sediments, thus providing a home for coldwater fish including various species of trout, as well as insects such as caddisfly larvae. Typically the riparian vegetation along an upper reach stream forms a very narrow band, including species such as narrowleaf cottonwood (Populus angustifolia), black willow (Salix scouleriana), thinleaf alder (Alnus tenuifolia), boxelder (Acer negundo) and red-osier dogwood (Cornus stolonifera). Although there is considerable overlap among birds that use lowland and upper-reach riparian areas, some such as the American dipper, are unique to these fast water systems. The proximity of these streams to mountain habitats means that some different species of mammals, such as elk and black bear, are typical here.
