Briefly, Monbiot makes two particularly pertinent points:
First, land with trees growing on it can absorb 67 times as much water as open pasture. So the best flood defence is growing trees. This is particularly important in upland areas where the headwaters collect. Our government, and the EU, give farmers subsidies to remove trees on hill farms.
Second, no river can ever contain more than a small proportion of all the water in its catchment area. Dredging will cause the river to flow faster, hurrying it to the nearest 'urban pinch point.' At the same time it can make river banks prone to erosion, stimulating a further build-up of silt so that by the following year the flood plain may become more inundated.
You can still see the article here: