This section on watershed has introduced the most basic features of an exceedingly complex type of natural system -- the physical template, and watershed structure and function. In closing, examine the graphic of watershed dynamics below and think particularly about all the interactions occurring even in this simplified example. It is truly important to appreciate the natural processes at work, and how they are beneficial to our communities as well as our ecosystems. Even more, it is crucial to recognize how change affects watersheds and can jeopardize these benefits in very costly ways, when a normal change becomes great enough to be a change of concern.

Climatology, the science of climate and its causes, becomes important in understanding regional issues in watershed science (Figure 8.4.18.4.1). Though sometimes used synonymously with weather, climate is actually a distinct term with important ecological ramifications. Climate refers to an aggregate of both average and extreme conditions of temperature, humidity, and precipitation (including type and amount), winds, and cloud cover, measured over an extended period of time. Weather refers to present day environmental conditions; current temperatures and meteorological events make up weather, not climate. Long-term weather trends establish averages which become climatic regimes. Climate heavily influences watershed vegetation communities, stream flow magnitude and timing, water temperature, and many other key watershed characteristics. Geology is defined as the science centered around the study of various earth structures, processes, compositions, characteristics, and histories. Geomorphology, however, refers specifically to the study of the landforms on the earth and the processes that change them over time (Figure 5). Fluvial geomorphology, referring to structure and dynamics of stream and river corridors, is especially important to understanding the formation and alteration of the stream or river channel as well as the flood plain and associated upland transitional zone; this is a critical discipline for effective, long-term watershed management. Figure 3. The physical template determines watershed structure Figure 5. Geomorphology helps explain river and watershed form. Figure 4. Climatic factors One of the life-sustaining cycles we are most familiar with is the hydrologic cycle (Figure 8.4.28.4.2). This cycle is a natural, solar-driven process of evaporation, condensation, precipitation, and runoff.

Decomposition involves the reduction of energy-rich organic matter (detritus), mostly by microorganisms (fungi, bacteria, and protozoa) to CO2, H2O and inorganic nutrients. Through this process they both release nutrients available for other organisms and transform organic material into energy usable by other organisms. In lakes, much of the decomposition occurs in the waters prior to sedimentation. In the headwater reaches of streams, external sources of carbon from upland forests are a particularly important source of organic material for organisms and decomposition of microscopic particles occurs very rapidly. The bacteria and fungi modify the organic material through decomposition and make it an important food source for invertebrate and vertebrate detritivores, thereby reinserting these nutrients and materials into the watershed’s aquatic and terrestrial food webs. Decomposition is influenced by moisture, temperature, exposure, type of microbial substrate, vegetation, etc. Specifically, temperature and moisture affect the metabolic activity on the decomposing substrate. Nutritional value (as well as palatability) of the decomposing structure will also affect the time involved in complete breakdown and mineralization. Decomposition involves the following processes: The leaching of soluble compounds from dead organic matter Fragmentation Bacterial and fungal breakdown Consumption of bacterial and fungal organisms by animals Excretion of organic and inorganic compounds by animals Clustering of colloidal organic matter into larger particles The process of death and consumption, along with the leaching of soluble nutrients from the decomposing substrate, release minerals contained in the microbial and detrital biomass. This process is known as mineralization.

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). An example using these terms is included below.