The name of the river has been derived from the Iberian "Iri-sama" which means "that which surrounds the big city".

Most of these streams descend from the slopes of mountains like Peñalara, Siete Picos, and Montón de Trigo.

After passing through San Ildefonso and Palazuelos, it reaches the city of Segovia, which it surrounds in a closed meander.

Eresma river then runs northwest, passing near Hontanares, Los Huertos, Bernardos, Navas de Oro, Coca, and Villeguillo.

The Eresma River has a pluvio-nival regime, which means that it has a period of high water flow in the autumn and winter season in proportion to precipitation and a short, violent water flow in the spring when the snow melts.