Union pickets mounted a peaceful protest at Cross Keys swing bridge, Sutton Bridge, after walking out in the early hours of Sunday.
Two bridge operators and a supervisor are striking for seven days in a disagreement with the county council over a pay cut.
Senior council highways managers are now operating the bridge along with a contractor who has worked there for some time.
The union predicts there will be disruption to road and river traffic, but the county council remains confident that it can cope with both.
Neither side in the dispute agrees how much the men earn.
The union says the bridge operators face a £300 pay cut per month – and their supervisor will lose £400.
The county council says it is simply bringing the bridge men’s pay and conditions in line with every other council employee.
Bridge operator and shop steward Tom Street said: “We don’t just open the bridge – 10 per cent of the job is opening the bridge. The other 90 per cent is maintaining it to a high and reliable standard so that the service is available on demand. Bridge breakdowns are very rare. This does not happen by accident.”