2. THE SHAPE OF A RIVER

FORMATION OF A RIVER

Rivers are formed from runoff from rainwater or melting snow and ice (glaciers). In addition, rivers are fed by groundwater (which is also supplied by precipitation) and vice versa (infiltration/ exfiltration). The rainwater follows folds and cracks in the landscape and flows as tiny streamlets running down slopes until they meet and, merging together, slowly grow first into streams and then into larger and larger rivers. As such, running waters form the ‘veins’ of landscapes.

On its way down, the flowing water shapes the landscape by carving into bedrock a network of valleys and catchments, transporting eroded material that is deposited in the lower sections of a river, creating wide valleys and vast plains. In the lower flat sections, a river takes a winding route, creating meanders and oxbow lakes. Eventually, the rivers flow into a lake or a sea. Some of the rivers form deep estuaries at their mouth; others, carrying enormous quantities of sediments that slowly fill up the recipient (a receiving body of water), form vast deltas with a system of marshes, channels, and lakes. Such deltas are newly formed wetlands. The Danube delta is the second largest in Europe, and it is approximately 4150 km² large with an average height of land at 0.5 m above sea level.

The Danube River system consists of numerous mountain streams and rivers that get most of their water from rain and melted ice and snow. It forms mountain valleys in the Alps, Carpathian Mountains, Dinaric Alps, and Balkan Mountains, as well as vast lowlands such as the Pannonian Plain and the Lower Danubian Plain.

In a dry season, when there is not enough rain to sustain the water level, streams and rivers can become temporarily dried out. This is typical for mountain torrents. Also, rivers that flow in highly permeable sediments can temporarily dry out. There are also rivers that suddenly completely disappear into the ground: the karst rivers, which are very common in Slovenia. Water in karstic areas flows through cracks and channels in the ground and reappears as a karstic spring at some other place.

MORPHOLOGICAL CHARACTERISTICS OF RIVER SECTIONS

Generally, the gradient of a river from its source to its mouth, and along with it also all the environmental factors (physical, chemical, and biological) change gradually in one direction: along the course of the river.

When we look at a river, we can generally identify three distinct sections that have specific characteristics:

• In the upper section of a river, featuring steep slopes, the water current is relatively strong and fast. It erodes or excavates deep into the earth, washes away rocks and gravel, and creates narrow valleys.
• In its middle section, the slope is decreasing, and the water current slows down. Valleys are wider because, in the past, the river deposited large amounts of gravel, sand and other material to form valley floors. The river starts to braid: to make turns and to form sandy-gravel islands that later may be covered with grass, shrubs and (willow) trees.
• In the lowest section, the river starts to make wide meanders in the river’s sandy, clayey, and peaty flood plain. The water current becomes very slow. Because of the constant erosion of riverbanks taking away material from one riverbank and depositing it on another, meanders slowly move downstream. A river flows in a very complex riverbed with many parallel branches and islands. The accumulation of sediment dominates erosion, and the river discharges into a lagoon and/or a sea. If a river carries large amounts of material, it slowly fills up the shallow coast and forms wide deltas. This section is one of the richest parts of a river regarding biodiversity, because it is a habitat for numerous plant and animal species.

EROSION, SEDIMENT TRANSPORTATION, AND TRANSFORMATION

Rivers carry out geological erosion and transport sediments along their courses, therefore, they constantly change and modify their river beds. They cut their riverbeds into rock and other geological formations and carry crushed geological base towards a sea or ocean. There are two processes involved in the formation of a riverbed: mainly mechanical erosion, which is based on the abrasion of the bottom and the banks of a river and, to a minor degree, corrosion (chemical weathering), dissolving chemically soluble material. The degree of erosion depends on the nature of the geological substrate, the slope, the amount of water, and the speed and power of water flow. The discharge changes seasonally, and erosion is particularly triggered by floods.

The mean flow velocity decreases gradually from the source to the mouth of a river, which is the consequence of the decrease in the slope of the terrain along the longitudinal profile of a river. The discharge influences the depth and the width of a riverbed, as these are generally increased downstream. It also influences the rate of flow and the transport power of water flow: stronger flow increases the transport capacity of water. Along the river course, the grain size of transported sediments is constantly decreasing from boulder to gravel, to sand, and to mud.

Sediment transport and load consist of bedload and suspended solids that dominate during flood events. It is evident that geological material is constantly mobilized and transported downstream with no possibility of return.

RIVER CORRIDOR, RIPARIAN AREA, AND FLOODPLAIN

The depression in which river water flows is called a river channel. The river course is formed by tectonic movements, especially in a mountain area, but also with changes of the surface relief. The river corridor is the space that is needed by high floods. It is narrow in valleys and wide in flat lowlands. During normal (mean) and low discharge, the river flows in its bed, which is limited by its banks. The riparian zone is the ecotone linking the aquatic and the terrestrial area. The floodplain is a larger area in flat landscapes where the biota are adapted to more extensive periods of droughts and floods.

SIGNIFICANTLY ALTERED RIVERBEDS AND ARTIFICIAL WATER BODIES

The type of geological material, the morphology of a river, and the riparian area are significant factors for the purification abilities of rivers. A river, according to morphology, can be classified in five classes (hydro-morphological method in accordance to the EU Water Framework Directive (WFD)): 1 = near-natural, 2 = slightly modified, 3 = moderately modified, 4 = extensively modified, 5 = severely modified. In addition to natural water bodies, the WFD also distinguishes artificial water bodies. For our purposes, we will simplify the method and distinguish only three categories:

• 1. Unchanged (and slightly modified) water bodies are natural or near-natural rivers and streams that have intact morphological structures (riverbed, banks) providing heterogeneous flow conditions (riffle-pool sequence) and habitats for aquatic biota, mostly high biodiversity and good self-purification ability. (CEN class 1 and 2). Such rivers are referred to as the reference state of each river type. They should be preserved and protected.
  2. Significantly changed water bodies represent the watercourse sectors with hydro-morphological alterations through channel regulation, hydropower, navigation, agriculture, water supply, flood protection, sediment extraction, etc. (CEN classes 3-5). At a minimum, the river continuum and the lateral connectivity are disrupted, and the water regime is changed. All these engineering impacts affect the biota negatively. If possible, such rivers should be restored, at least partly.
  3. Artificial water bodies are created by human activity. They can be completely artificial (concrete canal), or they can be designed to function as a natural water body.

Europe’s surface waters, as well as transitional and coastal waters, are affected by major modifications, such as water abstractions, water flow regulations (dams, weirs, sluices, and locks) and morphological alterations, straightening and canalisation, and the disconnection of flood plains. These are called hydro-morphological pressures. Hydro-morphological alterations are changes to the natural flow regime and structure of surface waters, such as modification of bank structures, sediment/habitat composition, discharge regime, gradient and slope. The consequences of these pressures impair the ecological function of aquatic ecosystems (Figure 7) and its biodiversity (aquatic fauna and flora) and can thus significantly impact the water status.

INTEGRATIVE WATER PROTECTION

A river encompasses three criteria: (1) morphology (incl. sediments), (2) dynamic discharge (water quantity), and (3) water quality (no pollution). This is summarized in the ‘good ecological status’ (GES) and ‘good ecological potential’ (GEP) as defined by the EU WFD. As stated before, intact hydro-morphological structures provide healthy aquatic populations; this also includes the riparian zones and floodplains of rivers that are the interface between aquatic and terrestrial ecosystems. Accordingly, integrated water protection encompasses all three criteria as well as terrestrial ecosystems. This is called the scientific catchment approach or river basin management. Such integrative management needs the cooperation and coordination of all stakeholders in a river basin, to find a compromise between the differing interests. Basically, for ecological but also economic benefits, protection or conservation should have priority over restoration. If one of the above criteria is heavily disturbed, the GES/GEP cannot be achieved. To be concise: neither clean water in destroyed channels nor polluted water in intact channels is a healthy ecosystem; to achieve GES, clean water must run in natural channels.

For high ecological status to be achieved, the WFD requires that there are no more than very minor human alterations to the hydro-morphological quality elements. For good, moderate, poor and bad status, the required values for the hydro-morphological quality elements must be such as to support the required biological quality element values for the relevant class. For each of the four surface water categories, specific hydro-morphological quality elements are designed.