Although it belongs to the municipality of Garganta De Los Montes, it runs through an impressive valley, flanked to the south by the Sierra de Canencia and to the north by the terms of Gargantilla de Lozoya, Pinilla de Buitrago and Navarredonda.

It is undoubtedly the longest intensive preserve in Madrid, with two sections totaling one approximate length of 7.5 Kms.

In the upper part of the preserve is the section without death that is occasionally refilled with common trout and that has an approximate length of 3.5 km. This section goes from the wall of the Pinilla reservoir to the mouth of the river in the small dam. de Las Casillas, from which a disused old mill collected its name, which gives its name to the preserve.

Below this wall, up to the mouth of the river in the Riosequillo reservoir, is the intensive section, which is periodically refilled with rainbow trout and which has a approximate length of 4 Kms.