The borders of the Reserve enclose the last dozen of kilometers of the river Merse, the final stretch of the stream Farma and the area surrounding the confluence of the river Merse with the river Ombrone, south of which the protected area involves the territory of Grosseto. On the whole, the landscape is gentle, with hills which do not go beyond the 400m of height and large flat stretches, often cultivated, at the confluence of the watercourses. In the inland of Siena the Reserve extends on a part of the valley of the stream Crevolone, a watercourse situated among harsh mountains entirely covered with woods.