the pencil and brush are part of the same control. As AScension said right-click on the brush icon in the toolbar and you should see three options (brush, pencil, airbrush) - select pencil.

then click the little arrow to change the brush set and select 'Square Brushes' and choose/set a low number of pixels (no more than 3) and retrace your rivers. I'd do it on a new layer above your old river layer. Then you can just hide (or delete) the older river layer.