River Builders

Flowers of the Branched Bur Reed, Sparganium erectum

Riverside plants have long been recognised for their ability to provide food and shelter for a range of animals – overhanging willows create ideal nesting sites for Moorhens, Water Mint and the many flowered Purple Loosestrife, among others, provide a food source for insects. It is now recognised that riverside vegetation does more than just influence species composition. They directly engineer landform development along river margins. For example by trapping sediment plants can change the unit stream power (1). In some cases, when trees are involved, this can lead to the creation of river islands (2).

Branched Bur Reed, Sparganium erectum

However it is not just trees that are capable of engineering rivers. The Branched Bur Reed (Sparganium erectum) is an emergent, sedge like perennial of still and slow moving fresh water and is common in northern temperate areas (3). It is currently being assessed for its ability to accelerate channel adjustment in low energy river systems (4). The plant acts by retaining finer sediment, organic matter and plant propagules which is held in place as the plant grows. This then allows the plant propagules to germinate, leading to the appearance of new species, which in turn promotes further retaining of sediment, resulting in landform construction.

Fruit of Branched Bur Reed, Sparganium erectum

References:

1. Gurnell et al. 2010, Geomorphology 116 pp. 135-144
2. Gurnell and Petts 2006, Earth Surface Processes and Landforms 31 pp. 1558–1574
3. Sterry 2004, Collins Complete Guide to Irish Wildilfe p. 270
4. Gurnell et al. 2010 Geophysical Research Abstracts 12 EGU2010-5883