In fluvial geomorphology different drainage patterns are identified on the basis of the arrangement of river channels and their tributaries. A drainage pattern in which the main river is joined by its tributaries approximately at right angles is known by which of the following names?

Introduction / Context:
This question comes from physical geography, specifically the study of drainage patterns. Different patterns describe how rivers and their tributaries are arranged on the landscape. Recognising terms like dendritic, trellis, radial and rectangular is important for understanding river systems and for answering map based MCQs.

Given Data / Assumptions:

• The key feature described is tributaries joining the main river approximately at right angles.
• The context is drainage pattern, not rock structure directly, though rock structure influences the pattern.
• The options include names of several recognised drainage patterns: dendritic, trellis, rectangular, radial and annular.
• Only one of these patterns is typically defined by tributaries meeting the main stream at near right angles in a parallel arrangement.

Concept / Approach:
A trellis drainage pattern develops in regions of alternating resistant and less resistant rock bands that have been folded or tilted. In such regions, the main river often follows a straight valley, while short tributaries flow from the sides and join it at nearly right angles, somewhat resembling a garden trellis. Dendritic patterns look like the branches of a tree with no strong structural control, and rectangular patterns form when joints or faults guide channels at right angles, but the description "tributaries joining a main river at right angles in a lattice like arrangement" most closely matches trellis.

Step-by-Step Solution:
Step 1: Note the key phrase: tributaries join the main river approximately at right angles. Step 2: Recall the textbook illustration of trellis drainage, with a main channel and many short side streams entering it like the cross bars of a trellis. Step 3: Compare with dendritic drainage, where tributaries join at various angles in a tree like pattern, not consistently at right angles. Step 4: Compare with radial and annular patterns, which describe flow away from or around a central high respectively, not right angle junctions along a main channel. Step 5: Conclude that the pattern described is trellis drainage and choose that option.

Verification / Alternative check:
You can verify by recalling that exam guides often provide a table of drainage patterns with distinguishing characteristics. In these tables, trellis is associated with "parallel main streams with short tributaries joining at almost right angles in folded terrain". Rectangular is linked with prominent fault lines and right angled bends in the main stream course, which is a different emphasis. This match between the descriptive phrase and the trellis definition confirms the answer.

Why Other Options Are Wrong:
Dendritic drainage resembles the pattern of a tree with channels joining at many different angles, which does not emphasise right angled junctions along a straight main stream.
Rectangular drainage has right angled bends and joints in the main river itself, due to fractures in bedrock, rather than numerous side tributaries meeting at right angles in a trellis like pattern.
Radial drainage refers to rivers flowing outward from a central elevated point, such as a volcano or dome, not to perpendicular tributaries on a main stream.
Annular drainage forms where streams follow concentric paths around a structural dome or basin, again unrelated to the description in the question.

Common Pitfalls:
Students often confuse trellis and rectangular because both involve right angles in some form. The key distinction is that trellis highlights a main river with many short side tributaries joining at right angles, while rectangular highlights the main river itself flowing along joints and turning at right angles. Another pitfall is to think that any branching pattern is dendritic, without paying attention to angle regularity. Careful reading of the defining feature in the question will help you choose trellis correctly.

Final Answer:
The drainage pattern where a river is joined by its tributaries approximately at right angles is called the trellis drainage pattern.