Transport and Storage of Water, Energy, Organisms, Sediments, and Other Materials

Because a watershed is an area that drains to a common body of water, one of its main functions is to temporarily store and transport water from the land surface to the water body and ultimately (for most watersheds) onward to the ocean. But, in addition to moving the water, watersheds and their water bodies also transport sediment and other materials (including pollutants), energy, and many types of organisms. It is important when recognizing the transport function to also recognize temporary retention or storage at different locations in the watershed.

Concepts: Transport and Storage. As matter physically moves through the watershed, there are a number of terms which arise relative to various stages of cycling. Availability refers not just to the presence of an element in a system, but also speaks to the usability of a given agent. For instance, nitrogen gas may be plentiful in and around dam spillways, but N2 is not a usable form for most aquatic organisms, and thus the availability of nitrogen is compromised. Detachment refers to the release of matter from an anchoring point, and its subsequent movement. Transport, a process most evident in stream channels, involves the movement of a material through a system. Deposition refers to a given endpoint within a cycle. Integration refers to the assimilation of matter into a site or organism following depositional processes (see Naiman and Bilby 1998). (Example using these terms)

Transport and storage of water. One can view a watershed as an enormous precipitation collecting and routing device, but transportation and storage of water actually involves a complicated mix of many smaller processes (which are bolded in the following text). Even before precipitation reaches the ground, it interacts with vegetation. Trees and other vegetation are responsible for interception and detention of some of the rainfall, leading to some evaporation and also slowing the amount reaching the ground via throughfall and giving it time for better infiltration to groundwater (one form of storage). Saturation of soils, occurring when precipitation exceeds infiltration, leads to overland flow and, over longer time frames, drainage network development. The consistent flow of water in channels affects and shapes channel development and morphology in ways that seek dynamic equilibrium with the job to be done (moving water downstream). Recall also this module's earlier discussion of the longitudinal profile development of rivers and streams, and how upper, middle and lower zones of streams generally have very different forms to handle very different sets of functions, many related to transport and storage of water.

Transport and storage of sediments. Watersheds also collect and transport sediments as a major function. Sediment transport and storage is a complex network of smaller watershed processes, like the water processes described above, and actually is inseparable from water transport and storage. Sediment related processes mostly involve erosion and deposition, but sediment transport and storage also play a longer-term role in soil development. The drainage network development and channel development discussed above appears to be dominated by erosion at first glance, but the redeposition of sediments on floodplains is an important function that rejuvenates soils and influences the productivity and diversity of stream corridor ecosystems.