Field Photos

Interfan area >> Climate/Hydrology >> Hydrology/Sediment transport



The interfan area is bordered by mountain-fed rivers (Gandak and Kosi River) and consists only of Foothills-fed, Plain-fed and Mix-fed rivers. To keep the comparability the mountain-fed rivers (alluvial fan area) are also listed.



Kosi river (mountain-fed)


Data from only one hydrological station on the Kosi are available, and that is at Baltara, very far down the main channel belt, a few kilometer above the confluence with the Ganga. In most years the highest monthly average of the daily discharges occurs in August. The monthly average starts rising in May, clearly preceding the monsoonal rains and reflecting snow and ice melt in the upland source areas.


Sediment concentration

The suspended sediment measurements at Baltara are low relative to the other rivers and are also remarkably constant with discharge suggesting a relatively uniform availability of sand, silt and mud grades of sediment.

Gandak river (mountain-fed) 

The discharge starts rising in early June, well before the arrival of the monsoon. This must reflect the melting of snow and ice in the high mountain source area that makes up about three-quarters of the system. The average monthly discharge is very similar at the two stations, which suggests that the alluvial course of the the gandak is a very efficient conduit for the water supplied. Although the peak discharge occurs in the month of August whereby the actual size of the peak discharges also shows great variation from year to year. The estimates for the discharges of the mean annual floods at the two stations show an increase downstream that presumably reflects input from the plains. And it is also interesting to note that whereas the mean annual flood at the upstream station is less than the bankfull discharge value, the mean annual flood downstream is greater than the bankfull value, implying spillage of floodwaters onto the floodplain there at least (on average) every second year.



Sediment concentration

The suspended sediment concentrations at various discharges have higher peak concentrations of fine material (wash load) upstream than downstream which probably reflects the generally somewhat greater frequency of flood waves upstream. Otherwise the ratio of coarse suspended material to fine is relativly stable at different discharges. The reeason for that might be the erady availabilitiy of coarse, medium and fine material in the channel system.



Baghmati System (Foothills-fed)


The discharge from the data (2 stations) shows a pronounced increase for June, when the main monsoonal rain arrive, so the contribution of snow and ice melt is insignificant for the Baghmati.

The mean annual flood estimates for both are distinctly greater than the bankfull estimations and this implies that overbank flooding (spilling) is a frequent occurrence. In its middle reaches, it is clear that the Baghmati spills westwards into the Burhi Gandak, and also eastwards in spill channels which take water into some of its tributaries, for example the Lakhandei, before the water rejoins the main channel belt. This spilling is a normal annual occurance, accorging to the people who live near the channel belt, and this is confirmed by the fact that the bankfull capacity of the river in the area is only one tenth of the peak discharge.


Sediment concentration

The monthly average suspended sediment concentrations show distinctly higher values for the wash-load at the relativly low discharges involved, compared with the mountain-fed rivers. This may reflect the prevalence of muddy banks in the channels of the Baghmati system. The total sediment load is very variable from one year to another and this may be due to the variability of the monsoonal rainfall in the rather small system area and the variability of erosion and deposition in the avulsing channels. (to get more details  click here)



Burhi Gandak River (Plains-fed)



Hydrological data from three stations in the Burhi Gandak system are available. The most upstream of these (Chanpatia) has much lower maximum of the monthly averages of the daily discharges because it is on a tributary to the main channel reaches of the system. All stations show an increase in the monthly averages of June, reflecting the onset of monsoonal rainfall and an absence of upland snow belt influence. It is also notable that peak discharges tend to be more variable from year to year at the downstream stations. At the most downstream of the stations (Rosera) the bankfull discharge measure is less than half of that at the midstream station (Sikanderpur) near Muzaffarpur and bankfull flooding is much more common at the midstream station.



Sediment concentration

Interestingly the downstream site has yielded higher concentrations of wash-load material in its suspended-load samples, suggesting the erosion of muddy material from the channel banks. Also the coarse/fine ratio in the suspended load tends to decrease with higher discharge figures, perhaps again reflecting the importance of local erosion of muddy banks. The total suspended sediment load tends to increase significantly from the midstream station to the downstream station and this further supports the importance of bank erosion within the system.



Kamla-Balan River (Mixed-fed)


The monthly average of the daily discharge data show typical June in creases, due to the start of the monsoonal rains, but a special feature in the data is that both stations show July an dSeptember discharge maxima. Both rivers also show considerable variablilitiy of peak discharge from year to year. At both stations, the mean annual flood is almost three times the bankfull discharge, which indicates the prevalence of overbank flooding.







Sediment concentration

The most striking feature of the Kamla-Balan system is the very high suspended sediment concentration that occur, despite the rather low discharge values even though the discharges of the mountain-fed systems. the Kamla, at Jaynagar, shows its foothill origin by having higher total suspend load concentrations and also higher ratios of course to fine suspended load, than the Balan at Jhanjharpur.



Additional morphological and hydrological characteristics of these rivers are compared in the table below. The most interesting comparison involves the sediment yield per unit area of the catchment. The Kamla-Balan and Burhi Gandak rivers have smaller average sediment load but their sediment yield is much higher than that of the Kosi river. The high yields are characteristics of the foothills- and plain-fed rivers. So not only the plains receive exceptional sediment influx from the high mountains, but once the sediment has been deposited on the plains, it is remobilized in an exceptionally vigorous way by the plains fed systems.


River Total catchment area
(103 km2)
Total length (km) Average annual discharge
(in m3/s) 
Average sediment load
(Mt yr-1)
Sediment yield
(Mt y-1 km-2) (x10-3)
Kosi 101 1216 2036 43 0.43
Gandak 46 630 1529 79 1.73
Baghmati 13 589 156 11 0.84
Burhi Gandak 12 579 58 15 1.24
Kamla-Balan 9 240 66 10 1.11