The Merse is a Tuscan river which originates in the Colline Metallifere, from the springs between Poggio Croce di Prata and Poggio di Montieri (848 m above sea level). It flows between the provinces of Grosseto and Siena, in a northeasterly direction in the first half of its path and toward southeast in its second tract, covering approximately 70 km through copse woods before flowing into the Ombrone at Pian di Rocca, near the springs of Bagni di Petriolo. In Pontiella the Farma stream, its tributary to the right which also originates in the Colline Metallifere (in Torniella) joins it; a few kilometers after it flows into the Ombrone as its tributary to the right.
The Val di Merse is the green valley spanning between Maremma, the Crete Senesi and the Val d'Orcia. It is a natural bridge between the Amiata and the Colline Metallifere, between the sea and Siena.
It is a body of water which flows through a beautiful backdrop of forests and monuments, including the splendid Gothic Abbey of San Galgano which dates back to 1218, that make it a beautiful itinerary for the people of Siena. Along its banks two natural reserves were established: the Alto Merse Natural Reserve and the Basso Merse Natural Reserve .
Worth noting are the “bathing” area of Brenna, one of the small internal lakes of Italy's ancestors, and the “mulino bianco” (white mill) of the well-known food industry of the same name, located on the bank upriver. The mill is the Mulino delle Pile, built in the early 1200s by the monks of the nearby Abbazia di Serena.