Trial results
The Scheme has conducted trials across seven canal systems. The Canal and River Trust has described the overall results as ‘encouraging, complicated, and in one case, ongoing.’
Summary of trials
| Canal | Duration | Outcome |
|---|---|---|
| Birmingham Canal Navigations | 6 months | Water quality improved by 40%. Two lock gates damaged. |
| Leeds-Liverpool Canal (Wigan stretch) | 4 months | Successful. Hippo refused to leave. |
| Regent’s Canal, London | 2 weeks | Terminated early. Narrowboat community dispute. |
| Rochdale Canal | 3 months | Successful. Hippo described as ‘professional.’ |
| Coventry Canal | 2 months | Suspended. Hippo lost interest near Nuneaton. |
| Kennet and Avon Canal | 5 weeks | Inconclusive. Hippo and swan reached what the Trust calls ‘a territorial understanding.’ |
| Stainforth and Keadby Canal | 6 weeks | Successful. Hippo cleared a section near Thorne that contractors had been unable to access for 18 months. |
Birmingham Canal Navigations
Duration: 6 months
The most extensive trial. Water quality improved by 40%. Aquatic vegetation was eliminated entirely. Two lock gates were damaged — the Trust describes this as ‘incidental contact.’ One narrowboat was wedged under a bridge in what the Trust has classified as ‘a misunderstanding about clearance height.’ The hippo appeared unbothered.
The Birmingham trial was the first extended deployment under the Scheme. The hippo, designated HCMS-01, was introduced to a 3km stretch of the Birmingham Canal Navigations near Tipton. Within the first two weeks, HCMS-01 had cleared approximately 800 metres of heavy aquatic vegetation, displaced a significant volume of sediment, and startled a family on a day hire boat in what the Trust described as ‘a mutual moment of surprise.’
By month three, water quality readings showed measurable improvement. Dissolved oxygen levels increased by 32%. The Trust’s environmental scientists described the improvement as ‘beyond our projected models’ and noted that the hippo had ‘achieved in weeks what a dredging programme would have taken months to accomplish, if the budget had been approved, which it had not.’
The lock gate damage occurred in month four. HCMS-01 moved through a lock chamber and contacted two gates. The Trust’s structural engineer assessed the damage and described it as ‘not catastrophic’ and ‘within acceptable parameters, if one defines acceptable parameters broadly.’ The narrowboat incident occurred in month five. The boat, moored under a low bridge, was lifted approximately 18 inches by displaced water. The owner described the experience as ‘unexpected.’ The Trust described it as ‘an unintended consequence of effective dredging.’
Leeds-Liverpool Canal, Wigan stretch
Duration: 4 months (nominally)
Vegetation cleared. Water quality improved. Hippo refused to leave after the trial concluded. The Trust describes this as ‘not technically a problem.’ The hippo describes this as nothing, because it is a hippo.
The full story of the Wigan hippo, including the timeline of events and current status, is documented on the Wigan Hippo page.
Regent’s Canal, London
Duration: 2 weeks
Trial terminated early following what the Trust describes as ‘a misunderstanding with a narrowboat community.’ The narrowboat community describes it as ‘an unprovoked aquatic intervention.’ The hippo describes it as nothing. The Trust has classified all further London trials as ‘requiring additional stakeholder engagement.’
The Regent’s Canal trial was intended to test hippo deployment in an urban environment with high recreational traffic. The hippo, HCMS-03, was introduced to a stretch near Camden. Within 48 hours, HCMS-03 had cleared a significant amount of vegetation from the canal surface, displaced several unoccupied moored vessels, and entered what the Trust described as ‘a standoff’ with a canoeist.
The canoeist, who was en route to a meeting in King’s Cross, described the encounter as ‘the most significant obstacle of my commute.’ The Trust noted that the hippo had not obstructed the canoeist but had ‘occupied the same space with considerably more confidence.’
The trial was terminated on day 14 following a meeting between the Trust, the Canal and River Trust’s London regional team, the narrowboat residents’ association, a representative from Camden Council, and a person who the Trust believes was ‘just someone who happened to be walking past and had quite strong opinions.’ The Trust has not ruled out future London deployments but has acknowledged that ‘the capital presents unique challenges, most of which involve opinions.’
Rochdale Canal
Duration: 3 months
The only trial described as ‘straightforward.’ The hippo was professional. The canal was cleaner. No property was damaged. The Trust has described this hippo as ‘the model employee’ and is considering a commendation. The hippo has not responded to news of the commendation.
HCMS-04 was assigned to a 2.5km stretch near Littleborough. The hippo maintained a consistent work pattern, cleared vegetation on schedule, and did not interact with any infrastructure in a way that required follow-up. The Trust’s site monitor described HCMS-04 as ‘the only hippo in the programme that I would describe as reliable, which is a sentence I did not expect to say when I joined the Canal and River Trust.’
At the conclusion of the trial, HCMS-04 was successfully relocated to a holding facility. The Trust described the relocation as ‘seamless.’ This is the only trial in which the word ‘seamless’ has been used.
Coventry Canal
Duration: 2 months
The trial was suspended after HCMS-05 showed what the Trust described as ‘a complete lack of engagement’ with the assigned canal section near Nuneaton. The hippo was introduced to a 2km stretch and proceeded to drift south without performing any discernible maintenance activity.
The Trust’s site monitor reported that HCMS-05 ‘appeared to find the Coventry Canal insufficiently interesting.’ When asked what would make a canal interesting to a hippo, the monitor replied, ‘I genuinely do not know. I have been monitoring hippos for eight months and I do not know what they find interesting. The Rochdale hippo seemed interested in everything. The Coventry hippo seemed interested in leaving.’
The Trust has classified this trial as ‘inconclusive’ and has not ruled out future Coventry deployments. A resident of Nuneaton wrote to the Trust to express disappointment that the hippo ‘did not even give us a chance.’ The Trust responded with a standard letter thanking them for their feedback.
Kennet and Avon Canal
Duration: 5 weeks
The Kennet and Avon trial was marked by what the Trust describes as ‘an unexpected territorial negotiation’ between HCMS-06 and a mute swan. The swan, which had been resident on the canal for an estimated seven years, refused to yield its territory. The hippo, which outweighed the swan by approximately 2,800 kilograms, also refused to yield.
The standoff lasted 11 days. During this period, neither the hippo nor the swan moved from a 50-metre stretch of canal near Bradford-on-Avon. The Trust’s wildlife officer described the situation as ‘the most British territorial dispute I have ever witnessed, in that both parties simply refused to acknowledge the other’s existence while occupying the same space.’
The trial was concluded after 5 weeks when the swan relocated to an adjacent stretch of canal. The Trust has described the outcome as ‘a negotiated settlement’ and noted that the hippo ‘did not acknowledge the swan’s departure, which is consistent with the hippo not having acknowledged the swan’s presence.’
Stainforth and Keadby Canal
Duration: 6 weeks
The most recent trial. HCMS-07 was assigned to a stretch near Thorne that had been flagged for maintenance for 18 months. The section was inaccessible to mechanical dredging equipment due to low bridge clearance and what the Trust describes as ‘a scheduling disagreement with the contractor.’
The hippo cleared the section in 6 weeks. The Trust described the results as ‘what happens when you remove the scheduling disagreement by removing the schedule and the disagreement and replacing both with a hippo.’ A local councillor described the improvement as ‘the most effective thing the Trust has done in this area, which is a low bar, but the hippo has cleared it.’
The hippo was successfully relocated at the end of the trial. The Trust has noted that this is only the second time a hippo has been successfully relocated, and that ‘two out of seven is a number we are choosing to view positively.’