Once the forest is gone, so is the soil...
The Role of Plants
Forests not only shield the earth by taking the full force of rainfall on their leaves, they also bind the soil together using their roots. Once the plant-cover is removed, the soil washes away quickly. This in turn makes it much more difficult for new plants to re-colonise the area.
Once exposed to the elements, rain quickly carves channels in the land, making the surface uneven and making it much more difficult to recover naturally, travel upon or farm. It also opens up a far larger surface area of sediments to be further eroded.
Effect on Waterways
Sediments can reach such levels in rivers that eventually light can no longer reach the bottom, stopping plant-life. It can make water inhospitable to river animals as well. Ultimately these sediments are carried out to sea, where they can then be deposited on Madagascar's coral reefs, smothering these delicate ecosystems.