1. by the cohesion of vegetative biomass to rivers



Riparian vegetation has a direct influence on fluvial
processes of braided river systems. Riparian vegetation can affect the flow
field, transport of sediment and fluvial morphology of these river systems. Braided
rivers are active and unstable systems that undergo constant change due to
varied rates of flow through multiple river channels. Riparian zones are regions
of transition between bodies of water such as rivers or floodplains, and the
surrounding upland terrain. These zones form an environment within which the
aquatic and terrestrial mechanisms interact and influence one another Naiman
et al, 2015. Vegetation growing within the riparian zones act as
bioprotectors; providing stabilization by the cohesion of vegetative biomass to
rivers sediments and protecting against the erosional forces of water. Fluvial
processes also influence vegetation growth and reproduction by affecting the
distribution and establishment of pioneer landforms that are created by the
deposition of riparian vegetation. The purpose of this paper is to look at the biogeomorphological
interactions between riparian vegetation and the fluvial environment. Understanding
these two-way interactions is important because vegetation is a vital
ecological control within fluvial environments, especially within complex braided
river systems.

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Overview of Fluvial Processes

Fluvial processes refer to the morphological
interactions between the flowing water and the channels of rivers.

Temporal and Spatial Scale of Fluvial Environments

Scale refers to the spatial dimension of a process or
feature with a system. One way to deal with spatial and temporal
scale in fluvial environments is to use a hierarchical classification system
within the context of geomorphic features, processes, and spatial boundaries Baptist,
2001. In the context of fluvial environments, using a system of hierarchy allows
for the organization of biotic and abiotic processes and interactions. The hierarchical
classification system, established by Christopher Frissell in 1986, is broken
down into the following levels; stream
segment, segment system, reach system, riffle system and microhabitat
system. Biogeomorphological processes can be found at several scale levels
Baptist, 2001. For the purpose of this paper, we are going to focus on the segment scale to evaluate river dynamics
and morphology of braided river systems and the reach scale to evaluate the riparian vegetation within braided
river systems.


The segment scale (river segment) is the portion of the
river basin defined by river pattern morphology, velocity and flow dynamics,
and river discharge. At this level, the spatial scales of processes and pattern
is quite large. River channel planforms can be defined at this scale and can
remain in equilibrium for hundreds to thousands of years Baptist, 2001.
Planforms are divided into four distinct categories; straight, meandering,
braided, and anastomosed. Straight and meandering planforms are normally
characterized as single-channel river systems, while braided and anastomosed
planforms are typically characterized as multi-channel river systems. The
planform type is determined by abiotic and biotic factors; sediment discharge,
slope, grain size, velocity, width/depth ratio and vegetation cover Baptist,


The reach scale (river reach) is the portion of the river
segment between breaks in channel slope, local side slopes, valley floor width,
vegetation and/or bank material Frissel et al., 1986. At this hierarchical
level, the spatial and temporal scales of processes and pattern is much smaller
than that of river segments, ranging from one to hundreds of kilometers in length
and between tens and hundreds of years Baptist, 2001. The biogeomorphological
interactions between biotic and abiotic processes is greater at the reach scale.
Vegetation increased the erosional resistance of river banks, which hinders the
development of meandering river channels.


Braided River Systems

Study Site – Taglimento River, Italy


Overview of Biogeomorphology

The field Biogeomorphology studies the two-way interactions
between the biotic (living) and the abiotic (non-living) world and encompasses
the fields of biology/ecology and geomorphology. This field is based on the
premise that the distribution of plants, animals and microorganism is often
influenced by the geomorphology and terrain morphology may be altered by these
organisms. There are three main biogeomorphological processes that explain
these interactions; bioerosion, bioprotection, and bioconstruction. These
processes have both active (direct) or inactive (indirect) forms that influence
their interactions with a living or non-living system. Bioerosion is the
chemical or mechanical erosion of land by organisms. In its active form, it
involves the direct weathering of removal of material. In its passive form, it
incorporates biological processes or agents which reduce protection, thus
indirectly encouraging erosion. Bioprotection is the prevention or reduction of
earth surface processes. Bioconstruction is the process of building,
accumulating or mounding of material by organic means. The material can be
internally produced by an organism, bind from other sources or develop from a
combination of the two.

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Biogeomorphological Processes of Riparian Vegetation

The above ground biomass of the vegetation alters the flow
field by retaining and accumulating sediments to form braided bars and other
landforms, whereas the below ground biomass affects the mechanical properties
of the river, creating a cohesive environment for other vegetative species to
thrive and adding protection from flood disturbances.

**Pioneer landforms as a biogeomorphological structure
within multi-channel river systems.



Characteristics of Riparian Vegetation

Plant species and structure of riparian vegetation is
strongly influenced by fluvial processes. For instance, pioneer plant species
that are the first colonizers of disturbed, bare alluvial sediment are capable
of quickly adapting to changes within the river system Gurnell, 2013. Woody
pioneer species within an active floodplain can colonize sand and gravel bars
created by flood disturbances Gurnell et al., 2001. These species are
hydrophilic (water loving) and can grow in nutrient deficient substrate. Riparian
shrub and tree species are significant for the river morphology dynamics
because of their large amount of biomass and their ability to act as ecosystem
engineers, driving the development of habitats that can be colonized by other
plant and animal species Gurnell et al., 2012. Salix elaeagnos (Willow) and Populus
nigra (Black Poplar) are two pioneer tree species that can survive flood
disturbances, drought and uprooting or burial of their vegetative structure
Gurnell, 2013.


They are known for their ability to reproduce both sexually
and asexually. Sexual reproduction of Willow and Black Poplar trees occur
through the wind and water dispersal of large quantities of seeds during the spring
and summer months. Asexual reproduction of these species occurs through the
sprouting of new plants from either vegetative fragments or uprooted trees and
shrubs left behind after a flood disturbance. Sprouting can occur anytime
throughout the growing season if environmental conditions are ideal and the
rates at which new plants can sprout is extremely high Camporeale et al.,
2013. In experimental sediment with a 30mm/day decline in the water table,
Willow trees had increments of daily average root depth of 27 to 20mm and Black
Poplars had increments of daily average root depth of 10 to 15 mm Camporeale
et al., 2013. This ability to rapidly grow fibrous root systems is critical
for new plant survival. This rapid growth allows plants to establish below
ground biomass and anchor swiftly into alluvial substrates, which enables
plants to resist uprooting caused by flood disturbances Karrenberg et al.,




Vegetation and Pioneer Landforms

Vegetation establishment can provide stabilization of
alluvial sediment by plant roots and create flow resistance, encouraging
substrate and seedling deposition which leads to the increase and spatial
extension of vegetated patches within fluvial systems Gurnell et al., 2012. There
is a complex interaction between the river morphological processes and the
vegetation morphological processes and biological traits. Riparian vegetation
has a crucial impact on sediment stabilization and the formation of biogeomorphological
structures, such as pioneer landforms. The formation of pioneer landforms can
be achieved in high energy river systems because of the rapid development of
both above and below ground biomass Gurnell et al., 2012.


Ecological Succession of Riparian Vegetation within River

Pioneer islands are initially formed by the deposition of a
riparian tree species, such as Willow or Black Poplar, that have the ability to
reproduce by sprouting new plant structures from the main vegetative structure that
has been initially deposited within the river system by a wind or flood
disturbance. Seedlings of riparian trees and shrubs can also establish in
alluvial sediments during periods of low flow or drought Camporeale et al.,
2013, Gurnell et al., 2012. As this new plant growth occurs, fine sediments
start to become trapped within the above ground biomass. The large biomass of
deposited woody vegetation and fibrous roots bind together with the river
sediment to form a stabilized pioneer island.


As pioneer islands grow, they can merge with other pioneer
islands, long-established vegetated islands or even unvegetated sediment or
gravel-bed islands, which can further stabilize the river corridor from erosion.
This aggradation and merging of vegetation creates species richness Camporeale
et al., 2013. As more sediment aggrades, the elevation of the island increases,
reducing the frequency of flood inundation. The pioneer island surface
environment begins to change; the deposited sediment becomes less disturbed,
dryer and more nutrient dense with organic material, thus creating a more
competitive environment for post-pioneer plant species to colonize the pioneer
island Camporeale et al., 2013. However, due to the unpredictable flow of
rivers, the ecological succession of vegetation with high energy river systems
can be interrupted and even reset by disturbances such as drought and flooding.



Research conducted on the reciprocal biogeomorphological interactions
between riparian vegetation within fluvial environments spans between the
fields of geology, geoengineering, geography, and ecology. There is a lot of
past and current research done of the interactions, using a variety of methods
from respected fields. Past methods for conducting research in fluvial
environments heavily involved field observations, creating laboratory flume
models and using repeat photography. While these methods are still used today,
advancements in satellite technology, and remote sensing and geographic
information science software has allowed researchers to use enhanced spatial
analysis methods of evaluating biogeomorphological interactions on a broad