Tirza - a right tributary of the Gauja in Cēsis, Madona and Gulbene districts. Length 80 km, basin area 754 km2. Annual runoff 0.18 km3. The fall is 157 m (2.0 m / km).

Tirza flows out of Lake Ezernieki in the central part of the Piebalga hill. In Gulbene district, Tirza flows along the border of Druviena parish, through Galgauskas, Lejasciems and Tirza parishes. Almost up to the Pearl (Liezēre - Druviena road) Tirza continues to meander through forested hills.

Then it reaches a wide valley, which is occupied by meadows, in some places also a forest. In several stages (at the mouths of Vilaune, Azanda and Virāne) Tirza has very distinct meanders with many old islands and places with small islets. Approaching Tirza, where Devonian rocks are found on the banks of the river, the river valley narrows and the meanders become slimmer. Devonian rock outcrops are also found in the section from Tirza to Galgauskas. There was a watermill near Galgauskas (Tirza HPP). Meandering processes are also very pronounced in the lower reaches of the Tirza at the mouth of the Lāčupīte.

Tirza flows into the Gauja near Lejasciems. The river is crossed by the Ieriķi - Gulbene railway line, Smiltene - Gulbene road, near Lejasciems - Smiltene - Alūksne road.

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