The physical complexity of rivers.

In the past, modifications to river systems were mainly conducted to prevent flooding and improve navigation. The extent of such influences means that completely natural river systems are now considered a rarity. These projects mainly involved straightening and deepening rivers, leading to physically simple channels. For example, the most globally reported of these issues is The Mississippi River, which was shortened by 235km and severely deepened across the twentieth century, reducing the overall health of the river. Modified channels often possess a uniform physical structure, with little complexity.

Figure 1 – Typical outline of a modified river system.

Modern restoration projects have combated the reduction in habitat quality induced by human interference by creating physical complexities within the channel. This involves altering the outline of the river by using techniques such as introducing meanders, changing the depth of the water and inputting vegetation and associated features (see woody debris).

Figure 2 – Example of a natural river system.

Changes in depth will naturally occur in all types of river due to changes in erosion and deposition of sediment. These processes lead to formations on the bed (bedforms). Different bedforms exist, ranging from small ripples like those seen on the beach, to mega dunes, like those seen in deserts that are created by the wind. Pools (deep zones) and riffles (shallow zones) are common bedforms and important to living things within the river system. Such features are crucial for fish, such as salmon, which often use riffles for spawning and pools for resting and eating.

Although inputting these natural features influences many aspects of the river system, a crucial (and easy to demonstrate) result is the change in flow. The following home experiment shows this.



Bibliography

 

Friends of the Issaquah salmon hatchery (n.d.) 'Fish habitat needs' http://www.issaquahfish.org/downloads/FISH-Learn-Unit_4.pdf

The RRC (n.d) 'River Skerne: restored 1995/98' http://www.therrc.co.uk/pdf/Publications/skerne_brochure.pdf

USGS (2012) 'A brief history and summary of the effects of river engineering and dams on the Mississippi River system and delta' http://therrc.co.uk/MOT/References/WT_Managing_woody_debris.pdf