As part of this PhD, a recent BGS survey has generated images of the lake bed as if there was no water.
This data has been used to produce a landscape map of the lake.
Windermere is the largest lake in the English Lake District measuring approximately 17 km in length with a maximum width of 1.5 km in the north.
It can be described as a glacial ribbon lake, which has been excavated and overdeepened by past glaciations.
The deposits from these glacial episodes have been erased during each successive glacial period and consequently the glacial deposits which remain are those of the most recent Devensian glaciation, dating between 75 000 to 17 000 years ago.
Ice retreated from the Lake District following the end of the Ice Age, marking the onset of a brief warm period known as the ‘Windermere interstadial’.
Following this period, ice re-advanced in the form of small valley glaciers during a cold period between 11 000 to 10 000 years ago, known as the Younger Dryas, or Loch Lomond Stadial.
Subsequent climate amelioration around 10 000 years ago marked the beginning of the Holocene period.
A multibeam bathymetric survey was undertaken in Windermere in September 2010 on the BGS research vessel (RV) White Ribbon.
A Planet Earth Podcast provides further details of the multibeam survey in Windermere.
The multibeam echo-sounder transmits sound energy and analyses the return signal (echo) that has bounced off the lake bed.
The time for the acoustic signal to travel to the lake bed and back to the receiver is measured and used to produce a detailed picture of water depths.
In addition to bathymetric mapping, multibeam data can be used for a range of scientific research, including mapping habitats and interpreting the geological and glacial history of landscapes.
In the future we hope to provide an updated bathymetric chart using our 2012 survey data combined with the 1937 survey chart; the new data, amongst other things, will fill in detail in the shallow areas.
The new chart will provide a useful resource to the local communities, the National Park lake wardens and other lake users.
In Windermere, the first survey of the lake bed was completed by in 1895 using a rowing boat, and a hemp line with a lead weight attached.
This was updated by an echo-sounding survey in 1937 which measured the water depth every 25 m along almost 300 east–west transects, spaced about 55 m apart.
These results led to the production of an Admiralty Chart, which was later updated in 1976 with corrected echo-soundings.
Collection of the swath bathymetry using the BGS multibeam echo-sounding system was based on these measurements and depth charts.
Further fieldwork in Windermere was carried out in June 2011, to collect samples of sediment from the lake bed. These were used to identify different sedimentary environments in Windermere, and are defined as: organic rich mud, finely laminated mud, gravel, fine sand, filamentous growths and bedrock.
Windermere is divided into two distinct basins, a larger North Basin and a slightly smaller South Basin separated by a line of low islands and shallows. The deepest point in the North Basin is 62 m, compared to 44 m in the South Basin.
The lake bed is characterised by a number of sub-basins separated by steps and ridges. Isolated areas of higher ground, plateau features and exposed bedrock further characterise this basin topography. These features were formed during the most recent Devensian glaciation. Superimposed on this underlying structure are debris flows and features formed by human activity relating to the Holocene period.
A number of debris flows are mapped on the lake bed. The largest of these is found in the North Basin and extends 450 m into the lake to a depth of around 45 m.
This debris flow moved downslope as a self perpetuating coherent mass before deposition beyond the slope break.
Potential trigger mechanisms of the debris flows in Windermere include:
The major debris flows in the lake have removed the surface drape of organic rich mud and have exposed underlying sediment.
We have recently acquired over 60 m of sediment core, 25 grab samples and 50 ROV (underwater video) transects of the lake bed. The sediment cores were collected using a floating platform and a piston corer.
The cores will place the major debris flows into the context of both space and time, and will be correlated with existing geophysical datasets.
The cores will also be logged and analysed through a multi-sensor core logger. This measures the physical properties of core samples without destruction. Radiocarbon dating will also be used to date horizons within the cores.