Complex sentence: Even extensive modification of the course of a river combined an enlargement of its channel often produce only limited reduction in the damage from its floods, consequently such floodworks are only commensurate with the expenditure involved where significant assets (such as a town) are under threat.
Split into simpler sentences: Even extensive modification of the course of a river combined with an enlargement of its channel often produces only a limited reduction in the damage from its floods. Consequently such floodworks are only commensurate with the expenditure involved where significant assets (such as a town) are under threat.