Talk:Llyn Trawsfynydd

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Size relative to Llyn Tegid/Bala Lake
The article suggests it is slightly larger than Llyn Tegid / Bala Lake, and hence the largest waterbody in Wales. It quotes 4.5sq km for that other waterbody but 4.8sq km is the figure given at Bala Lake. Any referenced figures around? thanks Geopersona (talk) 14:42, 19 December 2021 (UTC)
 * I now note that a slightly higher figure of 4.84sq km for Llyn Tegid / Bala Lake is given in the List of lakes of Wales article (as against 4.8 for Llyn Trawsfynydd) but that figure (like others at that location) is unreferenced. Geopersona (talk) 14:51, 19 December 2021 (UTC)
 * My own measurements suggest around 5.08sq km for Trawsfynydd which falls to marginally under 5.0 sq km if islands are excluded from the lake area, whereas Llyn Tegid amounts to 4.17 sq km. The discrepancy with respect to Llyn Tegid (on its own article page) is perhaps accounted for by the fact that the quoted figure comes from https://rsis.ramsar.org/ris/535 and, if the line on the map defining the extent of the Ramsar site is accurate then it rather exceeds the area of the actual waterbody, taking in land particularly at the southwest end. This is of course original research on my part so cannot be included in either article but does suggest what true figures might look like if and when an editor finds a suitable reference out there! cheers Geopersona (talk) 19:41, 19 December 2021 (UTC)


 * 'The Lakes of Eryri' by Geraint Roberts (Gwasg Carreg Gwalch, 1995) gives the size of Llyn Tegid as 1123 acres (4.54 sq km) and Llyn Trawsfynydd as 1180 acres (4.77 sq km). He describes Llyn Tegid as "Wales' largest natural lake".  Hogyn Lleol (talk) 19:58, 19 December 2021 (UTC)
 * I don't have access to a copy but if you do, you might cite it as a reference for those figures in the two articles. I'd still be wary of the accuracy of the Tegid figure but it's more accurate than what is there presently. diolch/thanks Geopersona (talk) 20:27, 19 December 2021 (UTC)
 * I too have copy of that book if there is any difficulty. As an aside, and not for inclusion here, the issue about Llyn Tegid and its "naturalness" or otherwise is complicated. The sluices constructed by Telford, and much later replaced, were never intended to increase the area of the lake by any significant amount. What they did do was to enable access to water for the canal, principally by allowing greater draw down. The average water level probably increased but not the maximum level. It is probable, and again this is OR, that the size of the original natural lake and the current lake are very comparable. The level and the area of the lake continue to vary  since the new sluices also permit the lake to be used as volumetric capacitance on the River Dee system by storing excess flow in high flow conditions and releasing it in low flow conditions. In practice this is a very sensitive mechanism and increases the efficiency of the system by balancing out the recreational discharges from Llyn Celyn and avoiding draw down of Llyn Brenig in all but the direst years.  Velella  Velella Talk  23:55, 19 December 2021 (UTC)