The Łyna is the longest river in Warmia and Masuria and the biggest of three rivers that flow through Olsztyn. The other two rivers are its tributaries.

The major part of its flow, starting with the source of the river on the slopes of the Lubawski Rise near Nidzica ("The source of The Łyna" nature reserve), coils its way through the territory of the Warmian-Masurian Voivodeship (approx. 200 km). The final section of the Łyna (64 km) passes through the territory of the Kaliningrad Oblast that belongs to Russia. This is also where the river, known there as Lava, flows into the Pregolya near Znamensk.

Olsztyn is the first city on the Łyna's course. However, before the river finally reaches it, it has to pass through several lakes: Duże Brzeźno, Kiernoz Mały and Kiernoz Wielki, Łańskie and Ustrych. Before the largest of these lakes – Łańskie – the Marózka flows into the Łyna from the left side. Beyond Lake Ustrych, the turning part of the river resembles a mountain river ("Warmian Forest" nature reserve).

The Łyna divides Olsztyn into two almost equal parts, while entering the city from the south, and leaving it in the northern part. On its course, it is contributed to by the water of lakes Ukiel and Kortowskie via the Kortówka Stream. Later on, it proceeds through the City Centre and along the Old Town, forming a significant element of the Castle Grounds landscape park. It flows under the oldest bridge in Olsztyn, the bridge of Saint John of Nepomuk, goes further near the Castle of the Warian Chapter and then under the railway viaducts from the XIXth century.
After leaving the city centre, it flows through the City Forest, where it is joined by the Wadąg, which flows into the Łyna from the right side.

Afterwards, it takes a northern course and crosses the territories of the Olsztyn Lake District and Sępopolska Lowlands. It flows through Dobre Miasto, Lidzbark Warmiński, Bartoszyce and Sępopol. During its further parts it is supplied by the rivers Symsarna, Pisa Północna, Gubra and Elma.

The Łyna is an attractive, international kayak trail, which allows learning about the region’s history and its natural features, which are particularly interesting within the areas of the aforementioned nature reserves. Dozens of kayak havens have been made along the trail. The trip can be started near Brzeźno Łyńskie and finishes in the frontier village of Stopki. In Olsztyn, short kayak trips through the city and its surroundings prove themselves to be quite popular.