River HMV Problems
Historically, rivers have been popular sites for township development, industries, and farming due to the available freshwater source, navigation, power potential, an fertile land. Whether for the protection of lives, property, or economical purposes, many modifications were made to the rivers channels and floodplains. These include the following:
Flow Manipulation
Reduced flow causes an imbalance in the system to which the morphological processes are reduces, and vegetation is no longer connected to the floodplain, and in turn vegetation can either die, or become out of control and alter the physical properties of the system.
Worldwide, flow manipulation is mostly caused by dam construction for hydropower generation, navigation, and irrigation draw. When the flow rate of a system is reduced, the streams ability to maintain the a natural dynamics are reduced or lost. In terms of hydraulics, the reduced volume, depth, and velocity reduces the river’s ability to mobilize sediment and encourage/regulate vegetation growth. Further, reduced flow rates reduce size and suitability of available habitat for riparian and aquatic ecosystems, induce/encourage channel incision. Upstream of dams, the flow manipulation causes large areas of inundation. While this will increase the overall water area, the river floodplains are lost. In severe cases, like the Upper Waitaki Basin, New Zealand, this inundation has drastically reduced the available habitat and furthered the endangerment of vulnerable species (i.e. the black stilt).
Sediment Manipulation
Dams not only store water, but they also store sediment moved from upstream. This sediment transportation disconnect between the upstream and downstream reaches causes significant problems. Within the trapped dam, sediment builds and reduces the available storage for water and causes issues with power generating turbines. Downstream, the river can become ‘sediment-starved”, where sediment is no longer delivering nutrients, building new habitats, and in some cases no longer removing pests (more on ecological benefits in Ecosystems and Habitats section). Reduced sediment loads and flow causes incision, which can cause large amounts of downstream sedimentation, severe bank erosion (which can lead to water quality issues), and lowering the water table.
In addition to the impacts of dams, other specific issues are constructed grade control structures, and sediment mining. While grade controls have been shown to reduce channel incision and bank erosion, some studies have shown grade control to exacerbate the conditions (Simon and Darby), reduce sediment transport, encourage sediment homogenization, and allow vegetation to encourage the active channel and thus reduce habitat and increase flood inundation levels.
Sediment mining is the removal of sediment from the river and floodplain and is often for the improvement of navigation, agriculture drainage, flood control, channel stability, or for construction aggregate. In relation to other mining operations, aggregate mining has both the largest volume and value in most States in the U.S. In certain cases of excesses sedimentation, sediment mining can stabilize the system. Moreover, this is not the case and sediment mining can cause channel straightening, increased incision, and downstream sediment starvation.
Vegetation Impacts
Deforestation, urbanization, agriculture, and exotic vegetation invasions are the causes to most vegetation alterations. These impacts can cause higher erosion rates, incision, higher runoff rates (leading to increased water pollution), less habitat, and altered channel dynamics (e.g. braided rivers can be corralled into single threads, deforestation can cause rivers to become less dynamic, etc.).
Previously, vegetation in relation to river systems was thought to exist simply because the river allowed it to; however, research over the last two decades has transformed that thinking to consider vegetation as ‘river system engineers’. Certainly, some vegetation can affect the fluvial processes that occur in a given river system through both above ground biomass (e.g. modifying the flow fields or encouraging sedimentation) and the below ground biomass (e.g. effecting the soil moisture and increasing soil strength through root reinforcement, which significantly impact soil erodibility). The interactions between vegetation and hydraulics and vegetation and morphology is not well understood, and not well modeled even in today’s cutting-edge computer models. Given the large number of uncertainty in vegetation physical parameters, soil conditions, local hydraulic phenomena, the ability to understand such interactions remains a large task for researchers.
While some vegetation species encourage healthy ecosystems, other invasive species can cause significant degradation to a river’s physical processes and ecosystems. Some of the most widely known invasive species include Tamarisk (Tamarix spp) (in the U.S.), Didymo (Didymosphenia geminata) (New Zealand, Australia, and parts of South America), Russell Lupins (Lupinus polyphyllus )(New Zealand), and himalayan balsam (Impatiens glandulifera) (parts of Europe).
