"mouth of the valley" is a correct and usable phrase in written English. You can use it to refer to a particular area of a valley where the water or the land is the narrowest or most concentrated. For example, "The mouth of the valley was a stunningly beautiful sight".
At the mouth of the valley lay Sils Maria, the one-time summer home of Friedrich Nietzsche.
Large volumes of transgressive marine sediment were delivered to the mouth of the valley by rising sea-levels during the early Holocene, forming a large subaqueous tidal delta.
A major checkpoint on the road was unattended, and we boarded buses at the mouth of the valley that took us into the heart of the settlement.
And, when we got to the mouth of the valley, all the wild sheep had come down from the hills and they were just huddled there together by the road, shuddering".
These rocks are seen in Chungthang area at the mouth of the valley.
In Koyadori, the mouth of the valley is closed by beach ridges.
The city lies 6 miles (10 km) north of the mouth of the Sous valley.
It lies along the Drava (Drau) River at the mouth of the Lieser valley, just west of Millstätter Lake and northwest of Villach.
The fighting unfolded near the outpost, known as Sisay, along a bad stretch of highway near the mouth of the Tangi Valley in eastern Afghanistan's rugged Wardak Province.
Wailuku, city, seat of Maui county, northern Maui island, Hawaii, U.S. It is situated on an isthmus at the mouth of the Iao Valley and the base of Maui's western mountains.
This zone is identified as the highest potential zone with discharges concentrated in an east west elongated area of 4 km2, near the mouth of the Puga Valley (Craig et al. 2013).
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK