I have two shapefiles, a polygon shapefile (lake) and a line shapefile (boudary) I want to create a new shapefile "polygon to the north of the boundary"
What tool can I use in QGIS or other free GIS for this task?
There is a QGIS plugin called Digitizing Tools:
The documentation says:
Split selected features with selected line(s) from another layer
applies to: line and polygon layer (multi or single part)
Splits all selected features of the active layer with the selected line features of another layer. The splitting creates new features (not multi features). Each new feature resulting from being split retains its original attributes.