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CHAPTER 2. RIVER ECOLOGY (*) 

(*) related to the MORPHOLOGICAL ASPECTS of FYR Checklist

River morphology

A river is an “environmental continuum”: a series of small environments and ecosystems that are full of life and transition from one into another. There is a so-called longitudinal continuity from theupstream to the middle reach to the downstream section of a river and a transversal continuity among theriver bed, the banks, and the catchments basin. The action (flow, erosion, etc.) of the river affects the riverbed as well as the nearby landscape, which is called an “area of riverine pertinence” which also includes the natural riparian areas (low marshy lands, willow groves, alder groves, hydrophilic meadows) along the river (question 2 of Morphological aspects list). In the natural environment of the river, the passage from the aquatic to the terrestrial habitat is not sudden but it is gradual within the riparian area.  This area has important ecological functions because it provides a buffer zone for nutrients (capacity of self-purification), an area of reproduction for fish, hydro-geologic regulation, increased biodiversity, climate regulation, and habitats for amphibians, reptiles, birds, mammals and many others (question 3-4).

River-bed

While crossing the river from side-to-side, one can recognize the ordinary river bed (the channel, which is occupied by the river in periods of normal water flow), as well as the low-water channel and the flood channel. The low-water channel has an indefinite and continuously reshaped pattern. On the other side the flood channel (flood area) is usually low, often covered with vegetation or cultivated and submerged during floods (question 5). 

Hours after a strong rainfall, the flow of the river suddenly increases. At the beginning, water is stopped by vegetation and percolates through the soil until refilling the aquifer, but when absorption surpasses the limit and the soil is saturated, water begins to flow to the surface and toward the river. This phenomenon explains the interval of time between the maximum rain and the peak flow. Conversely, it takes less time for water to flow from the sides of a river to the main course, if vegetation is scarce or even absent. In addition, during a flood, the increased flow and speed of the water increases the capacity of erosion and transport of inert material (question 6).

Figure 4: River morphology ( Colorado water resources CSU)

Transportation of material

Water is a geomorphologic factor (related to the form of the surface of the Earth). It sculpts the land, erodes the substrate and creates new shapes. The substrate (the stone, the soil, thesides and the river bed) offers passive resistance to the erosion of water and its course.

The water of a river has power (the kinetic energy produced by movement), as do all moving bodies. This energy is determined by the speed of the current and the flow. The friction from the deposits and structures on the riverbed dissipate a large part of this energy, which creates turbulence, eddies, and whirlpools, but the remaining energy is used to transport material towards the valley. The faster the river is, the more quickly the energy increases, and the larger the amount of the transported materials. In high slopes (usually streams), where the river is very fast, the material consists of small pebbles. In the stretches with a slight inclination (usually in the plains), where the river is slow, sand and silt are constantly transported. They can be carried in suspension, which is the case for especially fine material, over long distances, or by rolling along the riverbed as is the case for rough material.

During flooding a river’s flow and speed increase considerably, as well as its capacity to transport material in a particular stretch. This capacity is seen by the amount of material, or sediment, which is suspended in cloudy, turbid water and which gets caught in bridges, barriers, etc.(question 7)

Erosion

Energy that is not dissipated by friction and transportation, erodes other material from the substrate. Mechanic erosion in the river-bed is mainly caused by the abrasive capacity of the transported debris. Water acts directly on the river-bed (linear erosion) and on the whole basin (widespread erosion), especially on inclined and barren surfaces. The mechanic action of water on the substrate is important, as well as the action that solubilizes the mineral salts of the rocks and the action related to the alternation of frost and thaw (question 8).

Sedimentation

In stretches where the river is slower, the capacity of transportation changes. Transportation levels may drop, for example, because of a sudden decrease in inclination, or because of higher dispersion caused by friction with very rough ground. First, the rough and heavy material sediments out, while the fine material is carried along until it is “dropped” on the riverbed. Rough deposits form the upstream river bed. As the river flows toward the valley, its bed increasingly consists of smaller pebbles, sand, and silt (very fine deposits) (question 10).