1. 13.3 River Valleys
Headward Erosion: is a process of erosion that lengthens a stream, a valley or a gully at its beginning and also enlarges its drainage basin.

2. Canyon & V-Shaped Valley Evolution1. 2.

3. V-Shaped Valley Starts:1.

4. V-Shaped Valley Starts:2.

5. Stream Piracy – The Development of a Watergap

6. Before
After
Stream capture, or stream piracy, is a geomorphic phenomenon that occurs when a stream or river from a neighboring drainage system or watershed erodes through the divide between two streams and "captures" another stream which then is diverted from its former bed and now flows down the bed of the capturing stream. This can occur through headward erosion of one stream valley into another or through lateral erosion of meanders through the sediments between parallel streams.

7. An example!
Stage 1 - Beaverdam Creek, Gap Run, and Goose Creek flow eastward through the Blue Ridge and enter the Potomac.
(from Judson and Kauffman)

8. Stage 2 - As the land is eroded downward, the three east flowing creeks do not have the power to erode as far through the Blue Ridge as the Shenandoah, Potomac system. The Shenandoah extends itself southward by headward erosion through the relatively high land west of the Blue Ridge. It eventually captures Beaverdam Creek.
An example!
(from Judson and Kauffman)

9. An example!
Stage 3 - The capture of Beaverdam Creek added more discharge to the Shenandoah which was able to therefore erode more. Headward erosion leads to the capture of Gap Run. The water gaps where Beaverdam Creek and Gap Run used to flow through the Blue Ridge are left as wind gaps.
(from Judson and Kauffman)

10. An example!
Stage 4 - Eventually Goose Creek is captured as well. Snicker's Gap, Ashby Gap, and Manassas Gap are left as wind gaps. As the land on either side of the ridge is eroded down together with the ridge summit, the relative elevation of the wind gaps becomes higher and higher.
(from Judson and Kauffman)

11. Summarizing Erosion and downcutting through young horizontal units
Folded Units
Development of
water gap

13. Waterfall Formation

14. A waterfall is usually a geological formation resulting from water, often in the form of a stream, flowing over an erosion-resistant rock formation that forms a sudden break in elevation.

15. Some waterfalls form in mountain environments where erosion is rapid and stream courses may be subject to sudden and catastrophic change. In such cases, the waterfall may not be the end product of many years of water action over a region, but rather the result of relatively sudden geological processes such as landslides, faults or volcanic action.

17. Niagara Falls – Water action erodes weak shales at the base of the falls. This erosion undermines the tough dolostone layer above. From time to time, the dolostone breaks off, and the waterfall recedes.