Slash and burn agriculture is responsible for some sedimentation of the water supply. However a bulldozer or a backhoe does more irreparable damage in half an hour than can be done by our gardeners in a year. (Which is not to say that they should not be encouraged to adopt techniques that mind the soil rather than mine it.)
Silt and garbage in a river course do contribute to flooding but ultimately flooding is a measure of surface run off.
The earliest civilizations depended on the annual floods to return nutrients to their croplands and they knew enough not to build any permanent structures below the high water mark. In more recent times because of population pressure we have ignored this reality and Port of Spain is a case in point. When the Spanish settled in the area they named the river that ran along (what is now) Observatory St., across Park St., down Frederick St., across Woodford Sq. and down Chacon St., El Rio Tragarita. (Sp. tragar: to swallow or engulf; + ita: diminutive, i.e. "The Little one who engulfs.") This forty-foot chasm was in the habit of regularly washing large parts of the town out into the Gulf until Governor Chacon had its course diverted to its present route as the East Dry River. Yet the problem persists.
In a healthy tropical ecosystem, when rain falls the first thing it hits is an uninterrupted canopy of leaves sixty to one hundred feet above the ground. This breaks up the raindrops into a fine mist and large drops rolling off the leaves. These fall to the forest floor where they meet a one-foot thick layer of rotting vegetation and a tangle of roots. The water has no choice but to percolate down through the spaces between the soil particles until it meets bedrock where it forms the water table. Wherever the bedrock comes to the surface springs happen which are the sources of rivers.
In many watersheds in T&T the integrity of this system was severely compromised for the sake of growing tobacco, cacao, coffee and sugar cane to feed the addictions of the metropole, this lead to increased surface runoff. Some of these estates survived into recent decades and the secondary growth, where it exists, has re-established a healthy canopy. Bush fires caused by uncontrolled burning by gardeners and delinquents has reduced the ridges of some watersheds to fire climax zones of bracken, bull grass, banga and cocorite, leading to renewed increases in surface runoff. Now, hopefully, we are post peak of the last construction boom because, wherever the rain meets galvanize, concrete, pitch and manicured lawns there is 90 - 100% surface runoff.