LINKS FOR THE SCIENCE AND PRACTICE IN THIS COMIC >
- TNOC: Restoring Urban Streams, Ponds, and Wetlands
- TNOC: Mapping the Hidden Hydrology of Cities
- Title: The Sound of a River”. The title is based upon the saying that you “Can hear a healthy river” which is founded upon the idea that a healthy river should have a wide variation in flow pattern and dynamics, with a wide range of water sounds being generated by in-stream flow variation caused by water falling, changing speed and moving over or under obstacles such as stone or wood. In addition, the wide range of habitats created by the variety of flows in the river will increase biodiversity and so a listener will also hear the sound of the animals and plants living in or on the banks of the river, for example, insects, birds, fish, and mammals. Sounds could also be generated by the wind moving through bankside trees and vegetation. So, in general, the greater variety and volume of sound you can hear, the healthier and more natural the river.
- Image 7: Many rivers have been significantly altered to make them straighter, by creating new channels and their banks have been raised to prevent water from flowing onto adjacent fields. Most of this is relatively recent engineering work, done to speed up the rate at which water can be drained from land to the sea and facilitate the draining of land for modern agriculture. High-raised banks are in most cases originally created using material taken from the river bed and have the effect of disconnecting a permanent area of the river (the channel) from the seasonal channel or floodplain. Trees have also often been removed or significantly reduced to make it easier to maintain the channel and to prevent them from changing the flows once they fall into the channel. A river in the condition as that in this image is not in its natural state and the response to high flows will be significantly different from what would happen in a pre-modified natural river. The physical shape and properties of a river are as important as water quality and flow in determining whether a stream or river is biodiverse and healthy.
- Image 8: Willow trees are often associated with riparian (river banks) and wetland habitats and their roots can act to prevent erosion and stabilise banks. In some situations, where most other trees have been cleared, a few willows are left for this purpose and are managed by regular cutting to produce a distinctive shape. Left without intervention, willows will often spread easily and quickly, falling into rivers and streams, and can generate dynamic changes in direction and flow that add a range of habitat types to rivers. They are a valuable wetland tree, alongside alder.
- Images 15 & 16: Larger-scale, intensive drainage and river engineering work historically began as soon as powered machines were able to change large areas of landscape in a relatively short period of time. Rivers were managed to increase flows and provide efficient drainage through landscapes. In many places, this is still the dominant paradigm for managing springs, streams, wetlands, and rivers. The reasons for this drainage work have often been to allow different and more intensive forms of agricultural production. Many floodplains are extremely fertile because they have been exposed to nutrients from river sediments over many thousands of years. Ironically, to take advantage of that fertility using modern farming techniques and equipment in many cases requires the protection of that land from further inundation, the separation of the channel from the floodplain, and the creation of a homogenous channel, resulting in significant wetland habitat loss and loss of river functionality, leading to declines in many plants, invertebrate, fish, bird, and mammal species. In addition, preventing floodplains from performing their natural function, means more water flowing at higher rates towards villages, towns, and cities, which can result in an increase in flood risk for those areas.
- Image 21: A natural un-modified river will often occupy a large area of floodplain and include multiple threads or channels that change frequently in response to high flow events and the movement of sediments. Natural river systems often contain large quantities of living and dead wood, with smaller channels often having frequent log jams that will also drive dynamic physical changes in the channel. This variety of form and habitats, alongside large quantities of dead wood, means rivers and their associated wetlands are significant biodiversity hotspots. Rivers should be home to a large and diverse range of aquatic invertebrates, and aquatic mammals, in addition to fish and wetland bird species. High concentrations and abundance of plant and invertebrate biodiversity will attract a wide range of other bird and mammal species. Naturalised rivers will flow both within and outside their low flow channel, dependent upon rainfall, groundwater levels, and season. Floodplains tend to have higher rates of fertility, caused by the sediments carried from upstream which are deposited across the floodplain, and higher levels of moisture. Many farming techniques traditionally took advantage of the high fertility of floodplains, in some cases, even developing systems to artificially spread floodwaters through a pattern of fields. More modern farming systems, which use larger machinery and can increase fertility using artificially produced inorganic fertilisers, are not compatible with regular inundation, which is why rivers were and continue to be systematically re-engineered to prevent or reduce inundation. Some of the most fertile and easiest to work farmland can be found on floodplains. There is good evidence to show that re-connecting floodplains to river channels and restoring complexity and natural channels to rivers is a key mechanism for reducing flood risk to towns and cities.