convert a line layer into polygon about qgis_classical_studies

[PHAHouten]

The easiest way is to create a small buffer (few meters depending on the size of the river) around the line. Note that this will obviously not represent the exact position of the river. However, this buffer polygon can be edited with the vertex tool to make the polygon represent the river as close as possible.

[Schneider-Jan]

additional information about buffering spatial data: https://docs.qgis.org/3.16/en/docs/gentle_gis_introduction/vector_spatial_analysis_buffers.html?highlight=vector%20raster