When the land that contains a river at its base level is uplifted, it causes the river to erode the land more quickly in order to maintain its base level. As the river erodes the uplifted land, it cuts down into the ground, creating a new, lower base level. This process is known as downcutting.
As the river downcuts, it changes the shape of its channel. The channel becomes deeper and narrower, with steeper sides. This is because the river is now flowing through a smaller area than before, so it needs to flow more quickly to maintain the same discharge. The faster flow creates more erosive power, allowing the river to cut down even further.
Over time, the river will continue to downcut and erode the uplifted land until it reaches a new, stable base level. This new base level will be lower than the original base level, and the river's channel will have a new shape that reflects the changes in the land.