A considerable amount of spring water runs in the river up to the highland where it then begins to resemble a direct run-off river as it flows on.

The river surfaces in the Eyvindarstaðaheiði plateau and runs through Svartárdalur valley, then continues between Neðribyggð and Reykjatunga where there is a waterfall called Reykjafoss.

It flows into the bottom of Varmahlíð and into the Héraðsvötn shortly before reaching Glaumbær.

[3] In old literature, the Jökulsá river, or part of the Héraðsvötn, is said to have flowed west along the slopes of Vindheimabrekkur and that the Svartá ran into the glacier lake at the point where the river changes its name.

[4][5] Most flow into glacier rivers, whose water, or rather whose river beds, look very dark, nearly black, in comparison with the grey-brown glacier water.