Caba Biodiversity Pack Rivers and Streams

BIODIVERSITY PACK HABITAT GUIDE RIVERS AND STREAMS

Rivers and Streams encompass an extremely broad range of habitats, from intermittent headwaters, to energetic upland brooks and sluggish lowland rivers. They are highly dynamic, shaped by the intrinsic characteristics of the catchment. Geology and climate dictate the natural flow, water chemistry, sediment and nutrient delivery regimes that determine the morphology, hydraulics and productivity of the river and the species assemblages it supports. They also shape the habitat types and land use across the catchment which further influence the river’s characteristics.

Freshwater and terrestrial wetland habitats part of the delivery for biodiversity and are formed by the flow and retention of within Biodiversity ecosystem services to be water in the landscape. Their nature is 2020 and the Rivers have strong safeguarded. determined by landform and hydrological Water Framework natural connectivity, pathways, the characteristics of the water Directive (WFD). RIVERS AND supply, and climatological and biological Across catchments, within the channel influences which generate a mosaic of action to enhance itself and with other STREAMS rivers, lakes, wet grasslands, reedbeds, our many and other habitats of various degrees of freshwater habitats types of habitat. IN A wetness and types of hydrochemistry. is intrinsically Connectivity between CATCHMENT linked; with works the river corridor and The UK Biodiversity Action Plan (published to rivers able to CONTEXT in 1994), described the biological resources benefit wetland floodplain supports of the UK which were identified as habitats, and vice and connects aquatic River networks are being the most threatened and required versa. Delivery on and wetland habitats highly diverse, with conservation action – our priority species a catchment scale natural processes and habitats. Detailed plans set out actions can take account of beyond the river’s generating repeated to protect and restore our threatened these synergies, and banks small-scale habitat wildlife, and work continues today, as a key can look to secure features (particles, riffles, opportunities to pools) nested within achieve biodiversity progressively larger © Peter Cairns 2020VISION benefits across the full range of habitats scales (river segments, reaches, tributaries, present within a catchment. main stems). From source to sea there is an overall WFD AND B2020 change in environmental conditions. SYNERGIES: VERY Hydraulic energy generally declines, temperatures increase, extremes of ACHIEVABLE alkalinity are moderated, nutrients accumulate, and dominant bed sediments Targets for Good Ecological Status under change from coarse to fine grain. These the Water Framework Directive focus on large-scale longitudinal trends are obscured the physico-chemistry (pH, temperature, by smaller-scale variations, where for nutrients) and biology (aquatic plants, example a gravel-dominated riffle gives way invertebrates, fish) of our rivers. Status to a low-energy silt-dominated pool. improvements under WFD can make a contribution to all targets under Outcome Rivers have strong natural connectivity, 1 of Biodiversity 2020 which calls for within the channel itself and with other DYNAMIC, INTERCONNECTED CHANNELS Priority Habitats to be maintained in types of habitat. Upstream, headwaters PROVIDE A RANGE OF HABITAT NICHES favourable condition, degraded ecosystems link to upland and lowland mires, whilst to be restored, and areas of importance downstream reaches link to the coast

1 BIODIVERSITY PACK: RIVERS AND STREAMS through the saline transition zone at the lamprey and bullhead), some are longer BROWN TROUT LAY THEIR EGGS IN head of estuaries. Connectivity between (as in dace and chub), whilst others are GRAVEL RIVER BEDS the river corridor and floodplain, through extreme and extend into marine waters both lateral channel movement and to exploit opportunities for feeding floodplain inundation during floods, (salmon, sea trout, shad and lampreys) and supports and connects aquatic and spawning (eel). wetland habitats beyond the river’s banks. Vertical connectivity with the underlying The natural habitat ‘template’ generates groundwater provides further habitat recognisable community patterns opportunities for many invertebrates, with at a larger scale. For example, plant some species confined to these zones. assemblages are distributed along river systems according to hydraulic tolerance, Across catchments, the condition and with high energy streams being dominated management of rivers can impact upon by attached algae (periphyton), mosses the biodiversity of and ecosystem services and liverworts, while submerged vascular provided by wetland habitats, and vice plants appear progressively as velocity versa, highlighting the need to consider the declines and substrate stability increases functional connectivity (existing and future) downstream. Fish and invertebrate between habitats when planning catchment assemblages follow similar patterns, restoration. dictated by hydraulic tolerances but also by associated shifts in food availability NATURAL ECOSYSTEM and hydrochemistry. Small streams are naturally lower in nutrients and are heavily © Alexander Mustard/2020VISION FUNCTION IN RIVERS influenced by riparian trees and woody material in the channel, generating a AND STREAMS different foodweb to larger and more open river sections downstream, where light At the reach scale, streams and rivers AT A GLANCE GUIDE and higher nutrient availability generate naturally form dynamic habitat mosaics RIVERS AND STREAMS higher levels of plant productivity. At including riffles, pools, cascades, the downstream end of rivers, salinity backwaters, ox-bow lakes, coarse and fine- gradients play an important role, with a grained substrata, marginal vegetation, Headwater streams make up nearly range of fish species exploiting the saline woody debris, exposed tree roots, cliffs transition zone for feeding and spawning. 70% OF TOTAL STREAM and river banks, and exposed sediments, in Great Britain. Many all associated with different water depths LENGTH Organisms also move laterally between are ephemeral, with characteristic and speeds. Fluctuations in flow alter the river channel and freshwater habitats environmental conditions in these habitats associated with floodplains, either during communities of invertebrates and over various timescales, some experiencing periods of higher flow (fish, molluscs, plants) lower plants strong fluctuations in velocity and some or by active dispersal (mammals, aerial providing refuge against high flows. Shingle insects). As a result, many species occur in The fauna of bars, marginal zones and ephemeral both running and standing waters in river- fast-flowing rivers sections of headwater streams become dry floodplain landscapes. Floodplain species tends to be highly diverse, for part of the summer. without aerial life stages (such as molluscs) with a rich array of CADDIS- are generally dependent on flooding Riverine species and assemblages are episodes to maintain populations across FLIES, MAYFLIES and distributed throughout river systems the landscape, colonising or recolonising according to this natural habitat variation, STONEFLIES suitable habitat as it becomes available. with distributions interrupted only by watersheds between catchments and Rivers are extremely diverse in the habitats 15 SPECIES OF BEETLE, natural barriers such as waterfalls, which they provide: some rare and restricted, have define the limit of colonisation. Habitat use is governed by the various traits Headwater streams make up nearly 70% been found associated with (anatomical, physiological or behavioural) of total stream length in Great Britain, and waterfalls and their splash-zones exhibited by individual species/life stages. in ecological terms can be viewed as the Organisms may move between small- essential foundation for healthy functioning scale habitats to find optimal conditions river systems. They are therefore the The fish assemblage of low according to the state of flow, seasons and subject of a separate factsheet in this energy rivers is dominated by stage of life cycle, and are often dependent series. Broadly they comprise higher- cyprinids adapted to slow-moving on the close juxtaposition of suitable energy, lower-nutrient channels, which or still water, such as habitat patches to do this. provide important linkages with the PERCH, springs and flushes that feed them. They ROACH and BREAM The adult stages of fish and some are home to characteristic communities of invertebrates migrate upstream for invertebrates and lower plants, including Tidally-influenced reaches – reproduction, thus countering the the scare mayflies and crane-files of where the river enters the estuary tendency of river flows to wash individuals ephemeral sections, and supply colonists downstream. The resulting eggs and and food to downstream river sections. – are essential to a range of species, juveniles are taken passively downstream including bivalves and worms in the flow to distribute themselves in High energy river sections are available habitat. Some migrations are characterised by coarse bed substrates, foraged by GREENSHANK relatively short (such as those of brook ranging from gravel to boulders, depending and REDSHANK

2 BIODIVERSITY PACK: RIVERS AND STREAMS © Richard Steel/2020VISION

feature. These plant assemblages provide conservation interest in their own right, and habitat for plant-dwelling fauna such as the nymphs of dragonflies, damselflies, and rare snails and stoneflies.

The fish assemblage of low energy rivers is dominated by cyprinids adapted to slow- moving or still water, such as perch, roach and bream. These species lay their eggs on submerged vegetation, which the fry are also dependent upon for shelter.

Many species of low energy rivers cannot persist without coarse substrates, so DIPPER AND GREY WAGTAIL FEED whilst a less common feature of these ON STONEFLIES AND OTHER RIVER INVERTEBRATES river sections, the processes that sustain them are essential in maintaining the full characteristic assemblage of freshwater species. Similarly, features that generate channel sinuosity provide the habitat on the sediment supply generated by the to many invertebrate species declines. complexity that allows fastwater habitat upstream catchment. The invertebrate Seasonally-exposed sediments can and exposed sediment niches to occur fauna of fast-flowing rivers tends to be therefore be seen as a dynamic set of within these lower energy sections. highly diverse, with a rich array of caddis- related habitats, shifting and changing flies, mayflies and stoneflies, molluscs such with natural riverine processes and Where there are relatively low levels of as river limpet, and the crustacean white- providing conditions for differing species fine sediment generated by the catchment, clawed crayfish. assemblages at different points in time. stable gravel substrates can still be These assemblages require suitable extensive in lower energy rivers. This is More stable larger substrates host adjacent riparian and floodplain habitat to the case with England’s internationally luxuriant growths of mosses, liverworts fulfil their life cycles. important lowland chalk rivers. The alkaline and river jelly lichens, particularly in waters and extensive gravels generate woodland or through humid gorges and Natural fish assemblages are characterised highly diverse invertebrate assemblages, ravines. Stoneflies and other invertebrates by brown and sea trout, salmon, bullhead luxuriant submerged and emergent plant inhabit the mosses, providing a valuable and stoneloach giving way to grayling communities, and abundant populations food source for dipper and grey wagtail. and fastwater cyprinids (dace, chub, and of salmonids and fastwater cyprinids. The Waterfalls and their splash-zones generate gudgeon) as current velocities decline. As uncommon fine-lined pea mussel is only rivulets and humid moss-covered rocks with salmonids, these species lay eggs in found in this river habitat. favoured by invertebrates – 15 waterfall (or on) the gravel of the river bed. beetle species have been found associated The lowest reaches of rivers, just as with this habitat, some rare or restricted Slackwater refuges (pools, back-eddies the river enters the estuary, provide a in range. The freshwater pearl mussel also and submerged tree root systems) are a profoundly different type of habitat that prefers stable gravel and cobble substrates, critical natural component of high energy is essential to a range of species. These where juveniles and adults can avoid being rivers. They provide temporary refuges tidally-influenced reaches are termed the washed out by spatey flows. during spate flow conditions, and habitat saline transition zone, where the purely niches for species not adapted to withstand In rivers with more mobile sediments freshwater river progressively acquires the fast currents. They also hold finer silts (active shingle rivers) the high levels of salinity regime of the estuary. This zone is and sands essential to species such as disturbance are hostile to most plants, often neglected in river conservation, not lampreys, whose juveniles develop in and attached algae adapted to exploit considered classically riverine nor classically silt beds. Riparian trees and large woody these conditions become more dominant. estuarine. It is typically of low species Seasonally exposed sediments provide material create scour pools and undercut diversity compared with wholly freshwater habitat niches for specialist invertebrates, banks, promoting sinuosity which is critical reaches because few freshwater species most notably rare or threatened ‘ground’ in providing these can withstand and ‘rove’ beetles. The beetle assemblage slackwater refuges. significant levels of of exposed sediments varies according salinity. However, In the lowest energy to the coarseness of the sediment (from it is an essential sections, extensive shingle to sand), so that different shoals The alkaline waters and spawning and deposition of finer can support different assemblages; over extensive gravels of juvenile nursery area sediments occurs, 200 beetle species use this type of habitat for estuarine fish and species that England’s internationally in England, feeding on emerging stoneflies, species including exploit these muds, mayflies and caddis flies. important lowland chalk smelt, mullets, bass silts and sands (such rivers generate highly and flounder. It is The more permanently exposed shoals as pea and unionid a highly productive of active shingle rivers are important for mussels), become diverse invertebrate environment, with early successional vegetation, including more prominent. assemblages and burrowing bivalves butterbur, tansy (host of the endangered Submerged higher abundant populations and worms of tansy beetle) and willow scrub, and birds plants such as water- tidally-exposed such as ringed plover and oystercatcher. crowfoots and luxuriant of salmonids muds foraged for Once exposed sediments become stable marginal vegetation by greenshank and enough for vegetation, their importance are a characteristic redshank.

3 BIODIVERSITY PACK: RIVERS AND STREAMS Under natural conditions, there is variety in bank profiles and the reduction fundamentally altering the size of the unhindered passage for migrating species of wetland riparian habitat, and bank river channel. Impacts are exacerbated by between the estuary and the freshwater reinforcement ‘fossilises’ the bank profile physical channel modifications, because river. The main river is linked to tidal creeks and prevents natural dynamic processes of habitat niches that provide refugia for fed by freshwater springs in the valley sides habitat generation and change. In shortened, species at high and low flows are lost. that themselves undergo a saline transition oversized, and constrained channels along their length. This is the niche of plant hydraulic energy is increased, eliminating From within the Catchment, sediment specialists such as brackish water-crowfoot, slackwater habitats and contributing to and nutrient runoff from inappropriate and of saltmarsh communities. channel destabilisation through enhanced land management impact on water quality erosion. Bank erosion and channel incision and in-river habitat condition. Non-native PRESSURES ON OUR can also result where the natural sediment species such as signal crayfish can impact supply is artificially cut off, such as when characteristic biological communities, both RIVERS AND STREAMS trapped behind upstream weirs and dams. through predation and direct competition, Alternatively where sediment supply is and through altering physical habitat such Rivers and streams are rarely completely heightened, a channel held in place by as riverbanks. Fisheries management physically destroyed in the way that a flood banks can become perched above the can either promote or put pressure on woodland or grassland can be, as water floodplain due to the accumulation of bed characteristic biological communities, falling on the catchment will always require material, again hampering natural river- depending on the intensity and nature of a route by which to drain away. However, floodplain interactions. management activities. river length can be dramatically shortened by channel straightening, often resulting in In impounded sections ephemeral habitats the loss of >50% of river habitat area, and such as exposed sediments are ‘drowned’, since natural water level recession through where completely culverted, any habitat SEDIMENT RUNOFF IMPACTS WATER value is mostly lost. the summer is prevented. Biological QUALITY AND IN-CHANNEL HABITAT movements are also prevented, with major Modifications such as channel engineering consequences for long-distance migratory and maintenance, flow modifications, and species (shad, lampreys, salmon, sea pressures from within the catchment, can trout and eel) and for shorter within-river then heavily degrade the quality of the migrations. At the coast,tidal structures remaining river habitat. eliminate the natural saline transition zone and its characteristic assemblage, Bed lowering for land drainage and flood often replacing it with an impounded and defence has removed vital coarse bed enriched freshwater reach. Rich margins sediments which are not readily replaced in of exposed mud that support burrowing many lower energy rivers. As river-bed levels bivalves and wading birds are lost. are dropped and flood flows are confined to the channel, often consolidated by Impoundments and abstractions both alter flood banks, hydrological connectivity with the natural flow regimes, exacerbating floodplain habitats is lost. Bank re-profiling or masking peak or low flows, altering during straightening results in a loss of temperature regimes, and sometimes © Ross Hoddinott/2020VISION

KEY PRESSURES ON RIVERS AND STREAMS

DRAINAGE: HABITAT LOSS: IMPOUNDMENT: Bed lowering for land Channel straightening Impoundments, such drainage and flood reduces river length, as dams, alter natural defence confines bank re-profiling flow regimes and the flood flows to the results in habitat loss, size of the channel, channel and impacts and tidal structures resulting in bank upon floodplain eliminate the natural erosion and a loss of connectivity saline transition zone habitat niches

MANAGEMENT: INVASIVE SPECIES: ABSTRACTION: The intensive Non-native and Abstraction of water management of invasive species can for supply and land within the river impact characteristic consumption alters catchment can impact biological communities natural flow regimes, water quality through through direct often exacerbating nutrient and sediment competition or the peak or low flows and runoff alteration of habitats affecting species

4 BIODIVERSITY PACK: RIVERS AND STREAMS KEY MANAGEMENT MESSAGES

• RESTORATION OF of river habitat function (for example, • ALLOWING LATERAL through the removal of weirs), taking NATURAL PROCESSES a longer term approach enables CHANNEL MOVEMENT Measures that restore natural processes – natural recovery to play the fullest role This is a key element in restoring natural flow, geomorphological and water possible. A long-term vision encourages the dynamic habitat mosaics on quality regimes – allow expression of a management decisions which are which characteristic biota depend, characteristic and self-sustaining mosaic more sustainable, particularly if the and allowing a natural response to a of river biotopes, and provide the best seemingly ‘immovable’ socioeconomic changing climate. In many cases, an opportunities for priority species to survive constraints of today may be resolved in enhanced ‘erodible’ river corridor, in a changing climate. Working with, rather the longer term. bounded by some form of resistance than against, natural processes is also to further movement (such as set-back more sustainable in terms of management • RATIONALISING CHANGES tree lines), within which the channel and therefore cost. A catchment-scale IN SPECIES DISTRIBUTION can move and create at least limited approach encourages practitioners to AND ABUNDANCE physical dynamism, is an attainable consider how the river system would ambition. operate under natural processes as a The current distribution of many rare foundation for planning restoration. (and more common) riverine species is • RESTORING LOW ENERGY limited as a result of previous habitat FLOODING IN SUITABLE AREAS • LARGE-SCALE loss or degradation. Plans for species PERSPECTIVE conservation and ecosystem restoration Restoring natural channel dimensions should therefore take into account the and removing flood banks in suitable Running water ecosystems are complex, (positive and negative) implications for areas will reconnect the river channel physically connected habitats, with species of the restoration of natural with its riparian zone and floodplain, conditions dependent on those both processes, and of climate change. benefiting a range of wetland habitats upstream and across the catchment. Suitable habitat needs to be maintained and species, and providing flood Restoring flow, sediment and water or created to prevent local or regional management benefits by delaying quality regimes is key – it is not only extinctions and to aid species recovery. peak flows to downstream areas of about addressing direct physical Direct management, including high flood risk. The re-creation of impacts on the reach of concern. reintroduction, can also be considered areas of wet grassland and woodland to assist in the transition to restored helps to naturalise sediment regimes • INTEGRATED THINKING environmental conditions. by trapping fine sediments carried by ON RESTORATION MEASURES flood waters, (although relict wetland • IN-CHANNEL habitat could be impacted by poor- Measures to restore one component of STRUCTURES quality floodwaters if nutrient and habitat integrity need to be planned with sediment loads are not also tackled). others in mind. For instance, the biodiversity Weirs and dams have a range of benefits of physical habitat restoration physical effects on river habitats, as • RIPARIAN may be short-term and limited unless well as blocking the free movement VEGETATION water quality issues are also rectified. of some species. To address these impacts, structure removal should A patchy mosaic of long and short • TAKING THE be the aim wherever possible. swards, trees and shrubs, and bare LONG VIEW Modification to minimise impacts is ground will be maintained by natural the next best option, preferably using a river processes. Riparian trees are Although physical intervention is often by-pass channel, permeable to relevant particularly important, providing essential to trigger natural recovery priority species as a minimum. habitat complexity (large woody debris, exposed tree root systems, scour holes), as well as a food source © Guy Edwardes/2020VISION (leaf litter is vital, particularly for specialist species in headwaters). They are also important in buffering the river corridor against adjacent land management and provide stability against extreme channel movement. Increasing riparian tree cover to 30% (or more where consistent with restoring natural ecosystem function) provides patchy light and shade to the channel, reducing the impacts of rising air temperatures, thus contributing to BANKSIDE VEGETATION PROVIDES HABITAT, AND BUFFERS THE RIVER FROM climate change mitigation. RUNOFF

• MANAGEMENT OF LARGE WOODY MATERIAL ‘Woody debris’ is a critical component of a naturally functioning river, adding physical habitat complexity, refuge and a specialised substratum on which organisms live and feed. It should be left in situ wherever possible, or pinned to the bank or riverbed where there is a public safety risk. Ideally, the river should be zoned according to risk and woody debris managed accordingly – for example, in low-energy headwaters, accumulated woody debris is at low risk of becoming dislodged. Greatest benefit is generated by material set across or obliquely to the flow rather than parallel to the banks, since it increases differential erosion and deposition. Passive restoration of woody debris (tree planting to generate native woody input over time) is preferable to active restoration (importing material) though introduction is a good interim measure whilst riparian trees (that will provide a future natural supply) are maturing. • ENSURE EPHEMERAL HABITATS ARE CATERED FOR Often-neglected ephemeral features like winterbournes, seasonally exposed shingle bars, riparian wetland zones FENCING TO PREVENT BANK EROSION and seasonally encroaching marginal CAN MAKE BANKSIDE MANAGEMENT vegetation house much of the DIFFICULT biodiversity of rivers. Natural seasonal flow and water level recession is essential for their maintenance, as is protection against maintenance works livestock provides the greatest scope or heavy livestock trampling. Wide and for characteristic marginal and riparian shallow banks contribute good levels of habitats to develop. ephemeral riparian habitat – this can be REFERENCES AND generated by natural geomorphological • UNDERSTANDING THE FURTHER READING processes. LOCATION OF EXISTING FRESHWATER BIODIVERSITY • A narrative for conserving • AIM TO AVOID freshwater and wetland habitats BANKSIDE FENCING To maximise the benefits of restoration in England work, and eliminate damage to priority A great deal of close bankside fencing or endangered species, it is important to • Climate Change Adaptation has been erected to protect river banks obtain a clear picture of the distribution Manual – Rivers & Streams from high densities of livestock. Whilst of local freshwater biodiversity, (indeed, benefitting in-channel fauna, it can this knowledge is legally necessary for • IPENS River Restoration Theme generate a ruderal bankside vegetation some species). Practitioners should Plan of low conservation value, eliminating take account of standing water, running short-sward plant species and bare water and wetland biodiversity. • For practical advice/case ground for characteristic invertebrates. It Specialist advice can be valuable; for studies: The River Restoration prevents a naturally functioning riparian example, work being undertaken by the Centre zone from developing since it permits Freshwater Habitats Trust to identify high intensity land use right up to the ‘Important Freshwater Areas’ could bank top, and so should be avoided CONTACT US inform local delivery. where livestock grazing intensity can be reduced. Where not possible, and  catchmentbasedapproach.org where any level of livestock grazing may damage critical habitats (such @CaBAforum as exposed shingle), fencing that is well set back from the bankside and @ [email protected] allows scope for occasional access by

Edited by The Wildlife Trusts on behalf of the CaBA Biodiversity Group 2018. Design: lonelycottage.co.uk. Header image P1: © Ross Hoddinott/2020VISION.