Climate change poses a number of challenges for rivers and freshwater habitats. Meanwhile, the current state of our rivers means that the ability of these ecosystems to cope with change or disturbance of any kind is severely compromised. A range of river restoration techniques may be implemented to help with both reversing the ongoing declines in biodiversity but also, crucially, enhancing the climate resilience of river systems. These twin aims may be achieved if work is carried out at scale.
As extreme rainfall events are becoming more frequent, huge peak flows and ‘1 in 100’ year floods for example are occurring considerably more often than their name would suggest. Large floods can be catastrophic for fish populations, with whole age classes of juvenile fish susceptible to being washed out. High flows also impact water quality, with increased erosion of soils and riverbanks leading to increased turbidity, gravel beds becoming smothered by silt and sharp peaks in acidity where rivers drain degraded peatlands or forestry. A broad range of modifications to rivers and the wider ‘riverscape’ exacerbate the impacts of these rainfall events.
River systems used to act as giant sponges, with water from rainfall slowly percolating into the river channel and meandering downstream. As many of the marginal habitats such as wetlands and floodplains have been drained, and rivers dredged and straightened, water now crashes down through the watershed, causing huge peak flows and flooding of towns downstream.
Where rivers are given space, they can hold far greater quantities of water upstream, preventing the flooding of homes or the scouring out of river habitats and species. Reconnecting rivers to their floodplains and allowing water to spill out where practicable into wetlands will reduce peak flows whilst restoring essential habitats. Promoting natural flood management is important.
Refuge from high flows for fish and other aquatic species can be created by returning structure to simplified and modified river channels. Instream woody debris can create shelter in itself, whilst also diversifying flows and leading to the creation of backwaters, bends, gravel bars or islands for example that will also provide refuge
One of the most noticeable effects of climate change is the increasingly warm and dry summers, causing prolonged periods of low flows and drought. Low flows simply mean less available habitat to aquatic species, though the quality of what remains may also be impacted. Issues with water quality may arise due to the lower dilution of pollutants, whilst fish may be at increased risks from predation. Deep pools provide refuge for aquatic species during low flows. As pools tend to form on bends or in the lee of instream structures, these have commonly been lost as a result of historic straightening and dredging of the channel.
Addition of woody debris to the channel diversifies flows in what might otherwise be homogenous sections of river. As a result, this can lead to the creation of scour pools, or the promotion of ‘lateral migration’ of the channel, gradually forming a bend where a pool will form.
Similar to the issues with flooding, as all the water from rainfall crashes down through the river catchment, it only takes a relatively short period of time without any rain before the landscape dries out. The creation of wetlands or reconnection of rivers to their floodplains allows water to be stored within the system and once again to slowly percolate into the river, maintaining baseflows during periods of little rain.
Higher summer temperatures lead as expected to higher water temperatures. Due to drought, rivers are more vulnerable to these changes as reduced quantities of water are able to heat up quicker. Atlantic salmon become stressed at temperatures above 22oC, which significantly impacts their ability to survive and reproduce, whilst for trout this occurs at only 20oC. Water temperatures can get high enough to kill fish and other aquatic life.
Many invertebrate species are particular sensitive to temperature changes and are likely to become fewer in both number and species with rising water temperatures. This will have a knock-on effect for a whole range of species from bats to birds and mammals that prey upon invertebrate species.
Where restoring the sponge like qualities of floodplain and wetland habitats will alleviate the issues of low flows, it would also alleviate water temperatures, providing a steady source of cool water to the river through the summer months.
A particularly effective way to cool water temperatures is to shade it. Trees and shrubs along riverbanks can vastly reduce the amount of direct sunlight reaching the water’s surface, reducing water temperatures by several degrees. Along watercourses where vegetation is lacking due to over grazing or there is a lack of trees then works such as erecting riparian fencing and planting riparian trees can help to reduce water temperatures to acceptable levels.
Habitat Connectivity & Climate Change
Though many fish species, with the exception of seagoing species, might struggle to migrate northwards, many other species dependent on rivers will be required to do so in response to a warming climate. To enable this distributional shift, the availability of a corridor of suitable habitat must be present. The restoration of river habitats, whether it is those that are instream or riparian, may provide crucial ‘stepping stones’ for species on their gradual shift northwards.
GFT undertakes various habitat restoration projects on local freshwater habitats to increase their resilience to climate change impacts. Some of the main projects are detailed in the storymaps below:
GFT have a 6 month paid internship available. This exciting opportunity will start in March 2023.