The catchment area of the Ajay River is long and narrow. The river course has remained more or less firm and there is no evidence of marked changes in the course. The bed slope of the river in different reaches varies considerably. The slope of the main river is flatter than that of the tributaries and slope in the plains of West Bengal is much flatter than that in its upper reaches. The Ajay catchment lies in the path of tropical depressions or cyclonic storms which form in the Bay of Bengal in the monsoon period and move generally in the north-west direction. Abnormally heavy rain spells are generally associated with these storms during monsoon. The average annual rainfall in the river catchment varies from 1,280mm to 1,380 mm; higher values being associated with hills. Nearly 75% to 80% of the rainfall is concentrated in the four monsoon months of June to September.

The important tributaries of the Ajay are Pathro and Jayanti in Jharkhand, and Tumuni and Kunur in Bardhaman district of West Bengal.

The river system is divided into three reaches from the point of view of floods. The upper reach is almost hilly, having comparatively steep slope. The middle reach is subjected to occasional flooding caused due to breaches and overtopping of the embankment during high floods in the river. Whenever the river is in high stage, flood locking of its tributaries takes place, which causes heavy spill on both sides of the rivers. Even during moderately high discharge the lower reach is the worst flood prone reach of the river and it suffers from very frequent inundation.