Many people associate the Glomma River with stable summer temperatures, but measurements from Fetsund show that the water temperature can drop surprisingly fast several times throughout the summer – from 17–18°C down to around 10°C within just a few days.
The graph above shows temperature measurements from the summer of 2023 at Fetsund Bridge. Several times during the summer, the temperature drops abruptly before rising again almost as quickly as it fell.
This is not caused by local weather conditions at Fetsund. The reason lies further north – in Lake Mjøsa.
When strong enough winds from the south or south-east persist over time, cold deep water is pushed up in the southern part of Lake Mjøsa. This colder water is then transported down through the Vorma River and into the Glomma. After roughly one to two days, the temperature drop can be felt all the way down at Fetsund and Sørumsand.
The phenomenon is an important reminder of just how dynamic open water swimming really is. Even in the middle of summer, river conditions can change quickly.
Through our work with GlommaDyppen, we have for many years closely monitored water temperatures and measurement stations throughout the river system. Based on these experiences, we have gradually learned to recognize periods where the risk of rapid temperature drops increases, allowing us to make necessary assessments and adjustments if the water becomes too cold.
Anton on our team is now developing a dedicated temperature prediction tool that makes it possible to monitor temperature trends and the risk of rapid drops throughout the summer. The goal is to make this tool publicly available online for the open water community after further testing during the summer.
What happens in Lake Mjøsa can therefore have a major impact on the water we swim in many miles further south 🌊
