Langdale, Bowderdale and Carlin Gill

Also within

Cumbria County YORKSHIRE DALES National Park

Condition (Natural England)

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Quaternary

Landscape processes: Fluvial

Description

Description Langdale, Bowderdale and Carlin Gill are three river valleys which are part of the Howgill Fells, which lie north of Sedburgh in Cumbria. The three valleys drain into the upper parts of the River Lune which flows south to join the sea at Lancaster. Langdale and Bowderdale are important fluvial geomorphology sites for several reasons. First, they provide an excellent record of Holocene landform development in the form of terraces, gullies, alluvial fans and cones. Radiocarbon dates suggest a major phase of geomorphological activity occurred around 1,000 years before present, forming fans and cones and also producing a range of sediments from debris flow deposits to fluvial gravels. Second, a major flood in 1982 created a documented suite of contemporary channels and alluvial fan deposits. The flood caused renewed hill slope erosion, massive deposition on many fans and cones and major channel changes involving shifts from single to multithread braided patterns. Individual alluvial fans are dominated by different sediment facies including true debris flows, stony debris flows transitional between debris flows and fluvial deposits, fluvially deposited cobble and boulder bars and sheet gravel deposits. Third, the area has suffered relatively little human disturbance and therefore provides an ideal opportunity for detailed study of the dynamics of the fluvial system. The Carlin Gill valley is a site of exceptional importance for fluvial geomorphology. It demonstrates a wide range of landforms related to the development of the fluvial system over three distinct time scales. Long term Pleistocene landform development associated with river capture features includes the open col at Blakenthwaite, the elbow of capture at great Ulgill Beck and the canyon section of Upper Carlin Gill, with waterfalls at the Spout and Black Force. Holocene landform evolution is represented by river terraces, gully systems and alluvial fans and cones. The latter features are particularly well developed at Grains Gill, along the middle reaches of Carlin Gill and at Blakethwaite. Deposition of some of the sediments has been dated using the radio-carbon method to the last 1,000 to 2,000 years before present and the associated gully systems are now largely established. The modern fluvial system is represented by active gullies and braided and single thread cobble to gravel bed streams. Rates of erosion, gully development and the relationship to channel behaviour have been monitored at this site since 1969. At Blakenthwaite, for example, reactivation of gullies and the associated dissection of a small Holocene fan has created a classic alluvial fan morphology. Many of the channels in the Carlin Gill system also reflect this recent activity and shallow, unstable braided channels typically occur downstream of major sediment supply points, e.g. active gullies. Carlin Gill, therefore, contains an outstanding assemblage of fluvial features which have been investigated in considerable detail and which contribute significantly to the understanding of how upland fluvial landforms and processes develop over different timescales.

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