Preparing for Windermere Winter

How Cold Weather Affects Lake Health Two people walking over the fells with snow on the ground and Windermere in the background

As winter settles over Windermere, a fascinating natural processes kick in. Beneath the glassy surface, the reduction in temperatures triggers a seasonal reset that brings new challenges for lake health, giving us the opportunity to consider the part we can play. Whether you’re a wild swimmer greeting the morning with a dip, or a dog walker exploring the shore paths, understanding what happens in winter helps us care for Windermere together. 

What actually happens to the lake in winter? 

Cooler water mixes with the lake: In summer, warmer surface waters sit on top of cooler, deeper waters  – like layers in a trifle.  This is called ‘stratification’.  In winter, surface water cools, becomes denser, and sinks, prompting mixing of water from top to bottom. This “winter overturn” releases deoxygenated deeper waters and redistributes surface-oxygenated water to deeper layers, setting the lake up for the next season and supporting fish, invertebrates, and the microorganisms that keep the ecosystem functioning. 

Algae slow down—but nutrients don’t disappear: Shorter days and colder conditions reduce algal growth, which can improve clarity. But nutrients (especially phosphorus and nitrogen) remain in the system, bound up in sediments.  

Wildlife adapts: Fish seek stable temperatures, often in deeper areas. Waterfowl use sheltered bays. Reedbeds and marginal plants provide crucial winter refuge, protecting plants and animals.  

Why cold spells can be both helpful and risky 

The helpful bit: Mixing boosts oxygen where it’s needed and helps reset the system after summer stress. Clearer water can benefit overwintering plants and reduce light-blocking algae. 

The risky bit: When lakes mix, the recirculation of deoxygenated water can cause oxygen levels to dip. For some of the upper lakes in the Windermere catchment, this can cause a flush of deoxygenated water, which can present a challenge to fish. Heavy rain events also increase the input of contaminants to the lake: grit and salt from roads, soil erosion from saturated fields, and mismanaged septic tanks are all vulnerable to storms. 

What wild swimmers can do this winter 

  • Time your swims wisely: Avoid swimming for 48–72 hours after heavy rainfall. Runoff can carry nutrients, bacteria, pathogens and sediment into nearshore waters, reducing water quality. 
  • Choose robust access points: Enter at designated or durable access areas to avoid trampling reedbeds and that precious lake shore habitat. 
  • Practice biosecurity: Check, Clean, Dry your kit (wetsuits, tow floats, booties) after every swim. This helps stop the spread of invasive species between sites. 
  • Be algae-aware: Winter blooms are rarer, but if you see scums, unusual discolouration or musty odours, avoid entering and report concerns to the Environment Agency. 
  • Prioritise cold-water safety: Acclimatise slowly, wear appropriate thermal protection, swim with a buddy, keep exits short and warm up carefully. Healthy swimmers are better neighbours to a healthy lake.

What dog walkers can do to protect Windermere 

  • Bag it, bin it – every time: Dog waste is a source of bacteria to the catchment. It’s a simple action to help make a difference. 
  • Keep dogs out of reedbeds and sensitive margins: These areas stabilise banks, filter water and shelter wildlife. 
  • Stick to paths, especially when the ground is waterlogged: Footfall on saturated soils accelerates erosion, sending sediment and nutrients into the lake. 
  • Check, Clean, Dry for dogs too: If your dog swims or paddles, give gear (including toys) a quick clean and dry to limit hitchhiking invasive species.  Remember to avoid water access if you’ve recently used ‘spot on’ type flea and tick treatments – they kill aquatic bugs too. 

Community actions that make a difference 

  • Reduce home runoff: Sweep rather than hose drives, use permeable surfaces, and direct clean roof water into soakaways where possible. This keeps clean water out of sewers, helping capacity. 
  • Mind winter chemicals: Use de-icing products sparingly and store them securely. Choose lower-impact alternatives where possible. 
  • Know your septic system: If you have a septic tank, ensure regular servicing and avoid overloading during wet spells. Small improvements stop nutrients and pathogens at the source. 
  • Join local monitoring and clean-ups: Citizen science and seasonal shoreline cleans help spot issues early and build momentum for positive change. 
  • Don’t feed waterfowl: Leftover food and concentrated flocks of ducks and geese increase nutrient loading from their waste. Enjoy watching; let nature find its balance. 

Why your actions matter – now 

Winter is the lake’s recovery season. By protecting shore habitats, limiting runoff, and making mindful choices in and around the water, we set Windermere up for a healthier spring – fewer blooms, better biodiversity, and clearer, more resilient waters. 

Together, we can make winter a turning point for Windermere’s health.