The River Drainage lines hold particular significance in the region: not only do they provide redeeming topographic breaks in the general flatness of the plian and provide sub-regional or even local uniqueness and individuality to the different parcels of land, but they also govern to a great extent, the human occupance of land, particulrly the agricultural land and settlements.
The drainage pattern is dendritic in general, and the general characteristic feature available throughtout the plains is that the rivers meet at acute angles, and several tributaries form parallel or sub-parallel lines to the main streams.
The Ganga is the mater stream and is the recipient of all water lines in the region. The regional drainage can be studied through the subsystems: (i) the Gandak system, (ii) the Kosi system, (iii) the Son system and (iv) the other tributary drainage to the Ganga.
The Ganga, a snow-fed Himalayan river and heavily laden with detritus, flows sluggishly west to east, through the central-southern part of the region, being closer to the Southern Uplands than to the Himalayan wall so that while its northern perennial (except a few) tributaries have a wider arena to carve out their flat valleys through their highly meandering courses, the southern tributaries meet the Ganga rather hurriedly because of steeper slope, particularly east of monghyr and west of Rohtas. The average gradient is 6 cm per km in the region. The flood-plain varies from 5 to 30 km or more and is known as Khadar including sometimes vast diara lands between the occasionally braided channels and also sand flats, all subject to anuual inundation. The Ganga Banks are infested with stable and high (10 to 15 meters) levees intercalated with Kankar, gravel or other resistant rock reefs as at Patna, or with the norternmost projections of hard rock basements of the southern uplands, e..g., quartzites at Monghyr : sometimes the projection appear in the mid-stream as at Sultanganj. A such, the Southern bank is relatively more permanent and stable than the northern one.
Unlike large rivers, the Gandak does not receive any important tributary though it has a number of old deserted beds and distributary-like spill channels - the Banri, the Jharahi, the Daha, the gandaki, the Dabra, the Mahi, the Dhanuti, the Baya, the Saran etc. It has a higher gradient than the Ghaghara and is, therefore, more notorious for floods and changing courses. The channel has been contained in its 5-7 km flood - plain by protective embankments on either side.
In between the Gandak and the Kosi, the Burhi gandak is the chief tributary of the Ganga and receives the waters of the Baghmati.
The Kosi, formed by seven important Himalayan streams (Sapt - Kosi) in the eastern Nepal, receives no tributary in the plains because of its raised bed; it meets the Ganga at present near a little below Kargola. It is the 'sorrow' of bihar and the wildest and the most devastative of the Indian rivers, and flows through several capricious channels. Below the Chatra gorge, where the river enters the plains, there is a sudden break of slope. Unlike the Ganga and other rivers, "there is no sifficient space for the river to pass gradually through the stages of grading, maturity and aggrading old age. Here, after the impetuousity of youth, there is a sudden leap from the degraded to the aggraded stage with no graded interval -- a sudden change from the young valley in the mountains to the deltaic stage of building and uncertain shuffle in the plains. This is the most critical feature in the (peculiar) character of the Kosi." It deposits infertile sediments consisting of micaceous sands and renders vast fertile lands into sandy and marshy flats. The Kosi Project is an attempt to tame and train the river. A protective embankment from Bagha to its confluence with the Ganga has been thrown. The river has several distributaries like the Saura, the Kamla, the Barhandi, the Dhusan, the Tilabeh and its tributaries like the Sugarwa, the Balan, the Kamla etc.; most of these are seasonal.
The Ganga receives numerous tributaries from the Southern Uplands among which the Son is the largest. They have a markedly steep gradient (Son with an average gradient of 35-55 cm per km) with quick run-off and ephemeral regimes, being roaring rivers with the rain-waters in the catchment areas but turning quickly into fordable streams. These streams, being wide and shallow, become disconnected pools of water in the remaining part of the year. The channel of the Son is very wide (about 5km at Dehri) but the flood-plain is narrow, only 3 to 5 km wide. The river has been notorious for its changing courses in the past, as it traceable from several old beds on its east, but has been tamed squarely with the anicut at Dehri, and now more so with the Indrapuri Barrage, a few km upstream.
