A drainage pattern is described as discordant if it does not correlate to the topology [surface relief features] and geology [geological features based on both Endogenetic movements and exogenetic movements] of the area.
In simple words: In a discordant drainage pattern, the river follows its initial path irrespective of the changes in topography.
Discordant drainage patterns are classified into two main types: antecedent and superimposed.
Antecedent Drainage or Inconsequent Drainage
A part of a river slope and the surrounding area gets uplifted and the river sticks to its original slope, cutting through the uplifted portion like a saw [Vertical erosion or Vertical down cutting], and forming deep gorges: this type of drainage is called Antecedent drainage.
Example: Indus, Sutlej, Brahmaputra and other Himalayan rivers that are older than the Himalayas themselves. There are usually called as ANTECEDENT RIVERS.
Superimposed or Epigenetic (Discordant) or Superinduced Drainage
When a river flowing over a softer rock stratum reaches the harder basal rocks but continues to follow the initial slope, it seems to have no relation with the harder rock bed. This type of drainage is called superimposed drainage.
Usually, the drainage patterns (dendritic, trellis, etc.) are strongly influenced by the hardness and softness of the rock and patterns of faults or fractures.
Sometimes, however, the land rises rapidly relative to the base level of the stream. This increases the gradient of the stream and therefore, gives the stream more erosive power.
The stream has enough erosive power that it cuts its way through any kind of bedrock, maintaining its former drainage pattern.
You get a situation, then, where the drainage pattern does not correspond to the hardness or softness of the bedrock or to the locations of faults and fractures.
In other words, it is a drainage pattern which exhibits discordance with the underlying rock structure because it originally developed on a cover of rocks that has now disappeared due to denudation.
Consequently, river directions relate to the former cover rocks and, as the latter were being eroded, the rivers have been able to retain their courses unaffected by the newly exposed structures.
The stream pattern is thus superposed on, or placed on structural features that were previously buried.
The Damodar, the Subarnarekha, the Chambal, the Banas and the rivers flowing at the Rewa Plateau present some good examples of superimposed drainage.
Examples: The Damodar, the Subarnarekha, the Chambal, the Banas and the rivers flowing at the Rewa Plateau, rivers of eastern USA and southern France.
In simple words, the river flow becomes independent of present Topography. It flows in its initial paths without being influenced by changing topography.
Antecedent Drainage Cut through the newly formed landform and maintain the same path Himalayan Rivers.
Superimposed Drainage Cut deeper through the existing landform and maintain the same path Some medium scale rivers of the Northern and Eastern peninsular India.
Antecedent Drainage The soil formed is weak and it is easily eroded by the rivers.
Superimposed Drainage The rivers have high erosive power so that they can cut through the underlying strata.
Usually, rivers in both these drainage types flow through a highly sloping surface.