River Engineering To Create Better Trout Habitat.
Trout need a variety of different habitats within the river to complete a life cycle. Shallow riffles are needed for spawning and to oxygenate the water, whilst deep pools with abundant cover are necessary for protection from predators. This relates to the smallest upland stream and to the larger lowland rivers. If streams have not been disrupted by man (such as allowing stock access, building compoundments or straightening) these sequences appear naturally. Pools and riffles occur naturally in regular sequence within meanders and weirs.
Pools and riffles vary greatly along any river, their formation is dependant upon the strength of the river banks and the stability of the stream bed. Usually the stronger the bank the deeper the pools are and the more pronounced the meanders will be. As stated bank strength relies a lot upon the nature of the bank. Vegetation such as tree and plant roots all help to strengthen these areas. Therefore if rivers and streams have been altered and have become detrimental for trout survival river engineering is necessary. This ranges from bank revetment through to building shelters for trout.
7.1 Bank Revetment And Channel Narrowing For Trout Habitat.
Bank erosion is a serious problem in Ireland. This is due to the large variations
in flow throughout the year. 
Water levels can rise as quickly as they can
drop making establishment of many beneficial trees ans shrubs difficult.
Therefore large boulders are often used. These are placed along the riverside
and prevent the current from damaging the banks. The large boulders also
create excellent habitat for many species and offer good cover for trout.
Boulders placed at meanders of streams also help scour out deep pools.
Channel narrowing is not so much of a problem in Ireland where fewer rivers and streams have been widened and dredged for agricultural needs. However, overgrazing has widened some stretches and led to a loss of habitat. Often fencing in insufficient and natural regeneration of vegetation is not enough. Therefore it may be necessary to infill parts of the river. Cages filled with stone have been used, however these fail to grow cover and can be an "eye-sore". Mid- stream islands can be created and often the shallower areas regenerate with terrestrial vegetation over periods of time. Care must be taken though to judge the amount of narrowing carefully, over-narrowing creates the risk of flooding to surrounding land. Also care must be taken as to not direct the flow into risk areas a s mentioned previously.
7.2 The Use Of Current Deflectors In Creating Trout Habitat.
Current deflectors are used to scour pools and deposit gravel for spawning sites. A commonly encountered type of deflector is a downstream pointing peninsula known as a "croy" or "groyne". This type of structure is not recommended. Although it works under low flows, when over topped in high flows the current is dissipated sideways, causing bank erosion. Water tends to leave structures at right angles to the last surface it touched so a triangular rather than a single structure is recommended.
A single deflector concentrates the current against the bank scouring out pools. Often seen on upland trout streams is two deflectors which concentrate the flow towards the middle of the flow and prevents any bank erosion occurring. Deflectors are usually made from stone although on smaller streams logs can be used.
A common problem with river engineering is the snagging of debris which then causes obstructions. This is particularly relevant where forestry plantations are present as large amounts of debris can snag on constructions very quickly, especially when the river is in flood. The debris can stop current deflectors from working and clog up riffles, so care must be taken that no points are exposed where debris can collect.
Weirs can also be used to scour pools, however the geology leads to pockets of softer rock creating natural weirs which are excellent trout habitat. These in time can create water falls which may isolate fish but are excellent oxygenators of water.
7.3 Cover Creation For Trout.
Bankside vegetation is not always enough to create adequate cover for trout, and it also dies back during the winter. To overcome this, artificial cover structures can be used. These are very simply large wooden boards or half logs staked to the bank and river bottom. Or alternatively the boards can be attached by rope to stakes on the bank, these also fall and rise with the water level are significantly cheaper to construct. Good sites for such constructions are on the outside of meanders were the current is swift and the pools should be fairly deep.
These cover constructions can also be attached midstream if the banks are not suitable. Boards or logs should be pinned to the river with reinforced steel bars. The covers should be parallel to the flow where the depth is adequate to cover them fully at all variable water levels.
As mentioned previously care must be taken with these constructions that they will not collect debris, that will render them useless.
