The Roman Map of Britain Leucosetia? Lougosetia? River Mersey 

Leugosena  (R&C 248) next
Sethia Seteia vars. Seghia Segeia, SeghiaiV Segeiais, SeghiatiV Segeiatis (Ptolemy II 3 2)

    The only way to resolve these two forms is to consider Leugosena as two parts. The latter -sena is easily explained as an error for Setia. Leugo, the former, could be an adjectival leuco 'bright, shining, white'. Alternatively, Leugo might be a misspelled loc-o 'lake, pool', or perhaps a broader sense - 'a standing body of water'.
    Lougis 'marshy' is used in the name of the Island of Lewis Lougis Eirimon.

   Leug- could also be an error for Deua, the Dee. But, this does not explain the remaining -o-.