River structures - What influences them?

River structures - What influences them?

River structures are a product of underlying geology (in the catchment area) and climate, in particular discharge patterns. As you can imagine a river in a dry climate with seasonal heavy rains looks quite different to a river located in a temperate climate with regular rainfall.

  
Hydromorphology

When talking about river structures you will often come across the term Hydro-morphology, where hydro refers to hydrology (water related processes, such as precipitation and discharge) and morphology to geomorphology (geological processes, such as sediment transport, erosion and deposition).
In short
Hydromorphology = Water + Structure 

Furthermore the character of a river changes along its course and over time. Thus, you should be always aware of 3 dimensions (4 if you add time), when considering a river system: 

  • Longitudinal = following the river course from source to sea (or bigger river) 
  • Lateral = river – floodplain and surrounding catchment interaction
  • Vertical = river – river bed – groundwater interaction
A river is a continuum expanding in every of the above mentioned dimension.

 
Longitudinal


  

Every river undergoes certain changes along its course. These changes highly depend on the catchment area and are specific to every river. However, some generalizations can be made.

Starting at its source, rivers or streams are characterized by steep slopes and rough rocky sediment. The shape is narrow, often straight or only slightly bent, flow velocities are high and the flow is extremely turbulent. Gorges and deep ravines are often found in these upper reaches.

After some distance, this changes: slopes become gentler resulting in decreased flow velocities; finer material starts to dominate the river bed. The river widens and deepens with its increased discharge. In the lower reaches you will often find meandering rivers or rivers with large, lazy bends.

 
 Lateral


Often underestimated is the lateral connection between river and riparian areas. Sometimes it is only noticed during flood events when the river claims land along its river banks, but there is a constant exchange. Runoff and material enters the rivers via its floodplains and the groundwater table is normally directly linked to the water table in the river. Frequently you will find wetlands and marshlands in the vicinity of rivers, where groundwater leaks to the surface.

Whereas in upper reaches floodplains are often narrow and the lateral component is not quite so pronounced, in lower reaches it becomes more and more prominent.

 
Vertical

 

A natural river bed is not homogeneous. Deep pools are often next to gravel banks and are followed by shallow ripple structures resulting in a strongly varying river profile. Yet the river system does not stop at the bottom of its bed. It extends further in its vertical dimension. As mentioned above normally rivers and groundwater are connected and in constant exchange. The connecting surface is the so called hyporheic zone. When the water table in the river drops, groundwater exfiltrates into the river. When the situation changes and the water table in the river is high, river water infiltrates into the riparian groundwater bodies.

In dry climates it can happen that the water table in the river drops below the surface. Thus the river becomes an underground river, still following in its river bed but not being visible.

 

A river in its natural shape shows a high variety of different habitats. As shown above, the composition of habitats changes within the river system. Every habitat hosts specific animal and plant species. Read more about river ecology. Or read more about how to assess river structures.