Why rivers in Bihar floods every monsoon.

Bihar is India's most flood prone state especially the region of northern bihar. Northern bihar shares its border with Nepal if you look at the topography of bihar it is very much influenced by the flowing of both perennial and non-perennial rivers although the state has many rivers.

I will only tell you about those rivers that are flowing above the danger level due to which there are floods in northern part of bihar and it happens only during the monsoon period every year.

Gandak

This river originates in the northern Nepal near Tibet border from the Nubian Himal glacier in the Mustang region of nepal here this river is named as Kali-Gandaki river it flows through the entire breath of Nepal and enters into the Tehrai region of northern Uttar Pradesh and Bihar after entering India this becomes the Gandaki, now it enters India in such a way that the river meanders and flows along the Uttar Pradesh and Bihar border for about 30 to 35 kilometers before flowing fully into the state of Bihar the reverse course is in the south eastern direction where it heads towards the capital city of Patna and joins the river ganga at Sonpur before it becomes its left bank tributary now that you have understood the flowing pattern of Gandak river many nearby villages are flooded when the gandak river overflows.

Kamla

The Kamala river originates from churya range near sindhuli district of nepal and flows through Indian state of Bihar it enters indian territory in Madhubani district in bihar the river continues to flow south and finally joins river Kosi even this river is capable of flooding.

Bhagmati

The Bhagmati river originates high in the shef puri hills near the village of Bhagwar some 15 kilometers north of the Kathmandu valley it flows south and enters in the state of Bihar through Sitamari district the river flows south and drains into the kosi river even this river is capable of flooding.

Kosi

Also known as Sorrow of Bihar

It originates in southern tibet and northern Nepal border that means it is a transboundary river Kosi river is part of the Arun river it flows through the entire breath of Nepal and enters India to the state of Bihar after entering Bihar, it makes a long curve before joining the river ganga at Kursala of Katiha district now it is a known fact that Nihar is india's most flood prone state and it is because of river Kosi in fact the entire river has shifted from 115 kilometer westward due to the sedimentation and tectonic activities that's why the entire river forms a long sea curve before joining the river ganga otherwise the coast of the Kosi river is pretty straight forward from nepal to ganga but over the years it has shifted in this curve form and that causes heavy flood in this region anyhow by joining the river ganga river Kosi becomes its left bank tributary as I have mentioned river Kosi has shifted its course 150 kilometer from east to west during the last 200 years the reason behind the shift in Kosi reverse course is due to tectonic movement and also due to the rising volume of incoming water during heavy monsoon that's why the entire river forms a long sea curve before joining the river Ganga this entire region to the east of river Kosi is a flood zone even this river is capable of flooding.

                               (ISRO)

The northern part of Bihar is a plain area land elevation increases towards north and north eastern side that is why most of these rivers that come from Nepal enters into the state of Bihar due to the low land elevation and you also have to understand the fact that the northern region of Bihar and southern region of Nepal is a terai region meaning this area is at the foothills of the Himalayas you can also call it the north of the endogangetic plain that means it is a low land belt then obviously the rivers that originate in the Himalayan region will flow naturally towards the low land areas of northern bihar during heavy monsoon it rains heavily in northern Bihar as well as in Nepal due to that water level of these rivers increase when the water level of the rivers in Nepal increases obviously it will rush towards the lowland areas which is also known as the catchment area and most of that lowland area or the catchment area lies in northern part of Bihar villages that reside on the bank of all these rivers that i've mentioned faces heavy floods and finally all these rivers flow south and drains into the river Ganga which splits Bihar into two parts north bihar and south Bihar.