The Po has a drainage area of 74,000 km2 in all, 70,000 of those being in Italy, of which 41,000 is in montane environments and 29,000 on the plain.[2] The slope of the Po's river valley decreases from 0.35% in the west to 0.14% in the east, a low gradient. Along its path lie 450 standing lakes.[2] Almost all of the rest of the non-Italy basin is in the Italian speaking Ticino (Tessin) canton in Switzerland, and the outlying extreme south western section (again, Italian speaking) of Grigioni (Graubunden) canton lying to the south of the San Bernardino pass which forms the Po's watershed with the Posterior Rhine. The major tributary in question is the northern source half of the eponymous Ticino river (fed by its Moesa tributary in Grigioni) and the top third of the Alpine lake, Maggiore, formed by it.