Arroyo Espinillo is a slow-moving stream located in the Partido de Magdalena, Buenos Aires province, flowing approximately 30 kilometers before emptying into the Río de la Plata. It runs through low-lying agricultural land with a minimal gradient, and its channel is carved into Cenozoic sedimentary deposits composed predominantly of silty-clay sediments with abundant organic detritus. The banks are defined by gentle, low bluffs in places.

Water depth in the dry season averages around 60 to 70 centimeters at its deepest points, making it quite shallow for much of the year. During periods of heavy rainfall, depths can reach approximately one meter, at which point the stream overflows its banks and inundates surrounding fields. The stream is fed primarily by local rainfall and by springs along its middle section. Water clarity varies depending on the zone and the volume of water being carried at any given time.

The area near Atalaya provides one known access point, reachable from Ruta Provincial 11 at approximately kilometer 48. From La Plata, a route via RP 36 through General Mansilla and Estación Bavio connects to RP 11 heading toward the Atalaya area.

The stream supports a notable diversity of aquatic life, including fish from the families Characidae, Crenuchidae, and Cichlidae, among others. Species such as Astyanax stenohalinus, Bryconamericus iheringii, Oligosarcus oligolepis, Characidium rachovii, and Australoheros facetus inhabit these waters. Aquatic diversity is most abundant during the dry season.

Given the very shallow depths in dry conditions, paddlers should be aware that the stream may not be navigable throughout its full length depending on seasonal water levels. The low current and flat terrain suggest a calm, unhurried character, though the shallow, turbid water and soft substrate are practical considerations for trip planning.