Question

explain Lake formation

Answer 1

All lakes fill bowl-shaped depressions in the Earth's surface, called basins. Lake basins are formed in several ways. Many lakes, especially those in the Northern Hemisphere, were formed by glaciers that covered large areas of land during the most recent ice age, about 18,000 years ago

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Answer 2

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Rivers usually have two types of origin either through glaciers and they generally are perennial and rain-fed (seasonal where there are dry spells in a year). So this will give two types of river origin.

1As glaciers melt the water starts flowing through the least resistant part. Initially these water are in the form of small streams. These forms rills then gullies. As time passes their path gets deepened due to downward cutting. According to Davis theory, rivers have three stages young, mature and old.
In young stage rivers are mainly in mountainous region where due to slope rivers have huge energy. So they can carry huge load and participate in erosion. During this stage river is engaged in downward cutting through attrition, abrasion, collision etc. We generally find these types of features: V shaped valley, gorges and canyons, rapids, cataracts, waterfalls and plunge pools, river capture due to headward erosion etc.


2 In their mature phase we find that their speed decreases due to reduced difference in the base level and present river level of river. So the vertical erosion is reduced and lateral erosion predominates. This leads to widening of river valleys giving rise to U-shaped valleys. River cuts laterally and side walls of valley are eroded and plain area starts developing. Its load carrying capacity is also reduced so deposition of large rocks starts in river bed. General features seen are meanders and oxbow lakes, Alluvial fans and piedmont plains, reduced water divide, leeves etc
In old stage river has lost all erosional capacity and deposition is on peak. Prominent features are sluggish river flow, very wide channel, flattening of river, water divide eroded away, peneplain, boondocks and delta.

2 Same applies to river originating due to rain. but most of them are not perennial. But general mechanism of formation are same.