By law you must have an impoundment licence before you start to construct, alter, repair or remove an impoundment structure.

Some activities have little impact on the flow or levels of water even in low-flow conditions.  Where these impoundments have little or no impact on the environment and other water users, you may not need a permit for these activities. 

You can still apply for an impoundment licence if you prefer to have one.  You may need to do this for some accreditation schemes.

If you conclude that you do not need to apply to us for a licence, you should keep a copy of your checklist and supporting evidence (for example, before and after photos and measurements) for your own records.  It is evidence that you have carried out an assessment and documents your decision that you do not need to apply for an impoundment licence.  We may request that you provide a copy to us.

You may still need to apply for other consents or permits. Find out about other permits and permissions.

Before you start work on an existing structure you will need to find out:

  • if there is a licence already issued or going to be issued - we can help you do this
  • who owns the structure and seek their permission to do the work or take over an existing licence

Impoundments which don’t need a licence

There are several exemptions where an impoundment licence is not needed. These are:

  • works constructed without a licence before 1 April 2006, except when a notice is served under section 3 of the Water Act 2003 requiring application for a licence. Any work undertaken on these works after April 2006 will be subject to the normal licensing requirements
  • where the impoundment is authorised by a drought order or drought permit
  • where structures are authorised by an alternative statutory provision (for example an Act of Parliament)
  • where Natural Resources Wales serves notice requiring impoundment solely for the purpose of screening or passage for eels
  • the construction or alteration of impounding works within the district of an internal drainage board if (a) the works are constructed or altered by, or on behalf of, that board in connection with its functions, and (b) construction or alteration is commenced after the coming into force of The Water Abstraction and Impounding (Exemptions) Regulations 2017 on 1st January 2018
  • the construction or alteration of impounding works in a managed wetland system if (a) the sole purpose of the works is the management, operation or maintenance of water levels or flows in that managed wetland system, and (b) construction or alteration is commenced after the coming into force of The Water Abstraction and Impounding (Exemptions) Regulations 2017 on 1st January 2018.

Will your activity result in a change in the flow or level of the water?

You need to consider how your proposal will change water flow, quantity or level. 

If your impoundment, or amendments to or removal of an existing impoundment, could cause a change in the water level, quantity or continuity of flow, you will need to apply for an impoundment licence.

Impoundments, or alterations to impoundments, may:

  • alter the flow going downstream, so there might be less water available for others or the environment;
  • change the pattern of how the water flows over a structure;
  • change the water level and physical river features of a river;
  • change the physical form and processes of erosion and deposition of a river and its associated habitats;
  • make fish passage, either upstream or downstream, more difficult and so reduce habitat connectivity and ecological resilience.

For example, building a new weir, operating old structures which require any alteration (including rebuilding or removal of part or all of the works), or adding structures, such as hydropower turbines, sluices, penstocks and lock gates may cause a change in water level, quantity or flow.

Is your proposed activity low risk?

We recognise that some activities are considered low risk and have little impact on the environment and other water users and rights.

Good design and consideration when planning your proposed activity can ensure that it won’t cause detriment to the environment, protected rights and other water users. 

You don’t need to apply for a licence for low risk proposals.

If your activity is in or near a protected site, you will need to apply for a licence.

Check if you are on or near a protected site.

Examples of low risk impoundments

The examples below illustrate low risk proposals.

Some of the proposals in the list aren’t legally classed as impoundments but are frequently enquired about.

New activities

  • fish passes that have formal written approval by Natural Resources Wales and do not significantly affect the distribution of flow over the weir
  • placing a notch in a weir to encourage fish migration
  • placing (and where necessary securing) natural material, similar to that which would be found in the catchment (e.g. branches), across part of the channel to diversify flows, for example in river restoration projects.
  • fixed control or passive flood prevention structures within the channel that only hold back, slow the flow of, or divert, flood water (structures with movable gates or sluices will require an impoundment licence).
  • flap valves that only operate during flooding.
  • small, temporary (i.e no more than 6 months) measurement structures, such as v-notch or small rectangular weirs that will not accumulate a significant quantity of sediment.

Maintaining an existing structure

Maintaining a structure relating to the upkeep, care or preservation of the structure, but not increasing the structures dimensions or altering the upstream or downstream water level or flow. 

For example:

  • routine sluice maintenance
  • re-pointing (block work structures)
  • repairs, for example to expansion joint, stone pitching or bank/wall
  • void filling (in 'solid' structures)
  • managing vegetation
  • routine de-silting of part of the structure
  • re-positioning (rock, rubble or block work bank protection structures)
  • replacing elements that do not contribute to phased replacement of the whole structure
  • re-facing
  • skimming/covering
  • blockage removal, including screen clearing
  • embankment grouting.

Please note that some of the above activities or maintenance works that are not regularly carried out may require other permits and consents.

The removal or disturbance of sediment and silt can cause pollution and affect downstream transport of sediments.  To cause or knowingly permit any polluting matter to enter a watercourse, including silt and sediments, is an offence and may also be in breach of the conditions of any permits you hold.

Read more about pollution when working on or near a river on the Netregs website

Diversion works

Works that temporarily divert water in the immediate vicinity during construction or maintenance of a structure or other authorised in-channel activities and do not affect any other water users, rights or the environment. 

Other activities

Other activities which are considered low risk can include:

  • any activity outside the channel, for example on top of the river banks, such as installing or maintaining bridge supports or flood defences
  • work in the river, parallel and adjacent to the bank that doesn’t significantly narrow the channel, for example installing sheet piling, rip rap, block work and revetments for erosion protection
  • installing a wave wall on an existing reservoir dam that is not intended to impound any extra water
  • increasing the height of spillway sidewalls (that does not alter the impounding level)
  • altering, moving or constructing a spillway without increasing the impounded level or changing the downstream flow regime to the detriment of the environment.

Please note that all of the above activities are likely to require other permits or licences, including:

an Ordinary Watercourse Consent from your Local Authority

a Flood Risk Activity Permit from Natural Resources Wales

Land Drainage Consent from Natural Resources Wales.

Checklist to help you decide if your impoundment is low risk

Use the checklist below to help you decide whether you need to apply for an impoundment licence.

If you answer yes to any of the questions, you should apply for an impoundment licence.

If you answer no to all the questions or can provide supporting evidence or justification why your proposal will not have a negative impact, you will not normally need to apply for an impoundment licence.


Are there any existing abstractions or impoundments, including unlicensed ones, or applications which may be affected by the proposal?

Are there any other water users or riparian rights or interests that may be affected?

Will the changes impact water quantity or variability? For example, will the changes cause reduced flows over any part of the weir/structure?  Will it reduce flows or dry out the channel downstream of the structure?

Is the proposal likely to have an impact on a site or features of a Site of Special Scientific Interest (SSSI) or Special Area of Conservation (SAC), Special Protected Area (SPA) or Ramsar site?

Does the proposal affect protected species that may live at the site or elsewhere in the catchment?

Will the proposal impact on either the up or downstream passage of fish (including eels) in the river?

Are planned changes in river flow likely to cause significant change to invertebrate habitats?

Are planned changes in the river flow likely to cause a significant change in the macrophyte and diatom communities?

Will the proposal impact on any fish spawning or nursery areas?

Will pollutants be discharged or mobilised into the river during construction and/or operation?

Is the proposal likely to significantly increase river turbidity by increasing the quantity of suspended sediment, affecting water quality?

Will the proposal alter the geomorphology of the reach? For example, will the proposal substantially alter sediment movement or deposition above or below the site?  Will the proposal cause erosion of the riverbanks up or down stream?  Or cause scouring of the riverbed?

Does the scheme involve modification or construction of a new raised reservoir with the capacity of 10,000 cubic metres or more?

What to do with the completed checklist

You should keep a copy of your checklist and supporting evidence (for example, before and after photos, measurements and any specialist advice received) for your own records.  It is evidence that you have carried out an assessment and documents your decision that you do not need to apply for an impoundment licence.  We may request that you provide a copy to us.

Emergency impoundments

A very limited number of impoundments may be constructed or altered in an emergency before a licence is in place in order to prevent an immediate risk of

  • death, personal injury or harm to the health of a human being
  • damage to property or
  • damage to the environment

Such works are only exempt from licensing when undertaken by i) Natural Resources Wales (NRW) as an enforcement authority under section 16 of the Reservoirs Act 1975, or ii) by or on behalf of, a navigation, harbour or conservancy authority in connection with its functions.

NRW must be notified of the emergency work within five days of the date it was started.  The notice must provide the reason that the authority or the person considered that an emergency had arisen and also confirm that the works were necessary to prevent an immediate risk to human injury or life, property, or the environment.

We may issue a retrospective licence to cover the works or serve notice that i) the emergency exemption was not relevant or ii) the works are no longer required to prevent immediate risk to human life, property, or the environment.

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