Geomorphological controls on tufa deposition at Nash Brook, South Wales, United Kingdom

Tufa deposition is an important short term influence on carbon cycling and an interesting, under-studied component of many karst landscapes. Nash Brook, in South Wales is a small, tufadepositing stream located in a small valley choked with older tufa deposits. Tufa is currently being deposited along a suite of small barrages within the river channel. Studies have been made of the morphometry and arrangement of these barrages, and short-term (9-month) measurements of tufa deposition made across a sample of the barrages. The morphometric studies show a complex series of relationships between barrage height, spacing and slope gradient, and allow hypotheses about hydrological controls on barrage formation to be tested. Large woody debris appears to play an important role in the initiation of barrages, and various microflora (including the alga Vaucheria geminata) aid the small scale accumulation of tufa. The highest short-term rates of tufa deposition occur on barrage crests towards the headward end of the barrage system. Current tufa formation at Nash Brook appears to reflect the interaction of hydrological and biological controls. Although small in magnitude compared with the fossil deposits in the valley, tufa barrages continue to play an important role in this karst fluvial system.