The Quaternary history of Llangorse Lake: implications for conservation

Abstract 1. Llangorse Lake is a relatively recent feature of the landscape—a product of Late Quaternary glaciation. It is a relict of proglacial ‘Lake Llangors’, whose water level was once 37 m higher than that of the present lake. 2. Palaeolimnological studies indicate that the lake basin started to infill with lacustrine clay in the Late Devensian, in a cold, steppe‐tundra environment. An interlude of relative warmth (the Windermere Interstadial) may have prompted accumulations of lake marl, before renewed clay sedimentation in the Loch Lomond Stadial. In a period of increased warmth in the early Holocene, catchment soils stabilized, and an organic autochthonous nekron mud accumulated slowly in central areas, whilst marl deposits accumulated in the northern margins. 3. Sediment later accumulated faster, apparently after Neolithic and Bronze Age activity in the catchment led to an increased proportion of allochthonous material. A major change occurred in late Iron Age/Roman times, after which sedimentation in the central troughs has principally been of red‐brown silty clay, eroded from the catchment. 4. In the eastern trough, nearer the principal inflow stream, ca 4 m of silty clay has accumulated in the past 2000 years. The water depth has shallowed to some ca 7–9 m in the deeper parts, compared with >20 m after deglaciation, and ca 15 m at the start of the Holocene. (These figures exclude field evidence for the lake having been considerably larger, with much higher lake level, early in its history.) 5. Several lines of evidence show that the rate of infill has accelerated: in the eastern basin 90 cm of sediment has accumulated within the last 300 years, of which the top ca 65 cm (including less‐compacted recent sediment) accumulated in <150 years. Unless input of allochthonous sediment is curtailed, the lake will become too shallow to be sustainable over the remainder of the Holocene. Copyright © 1999 John Wiley & Sons, Ltd.