Trellis drainage pattern

Topic: Drainage System of India: Rivers, Basins and Patterns · NCERT: Class 11, Ch 3 "Drainage System"

Meaning

A trellis drainage pattern is a river pattern where the main (primary) tributaries run parallel to each other and smaller (secondary) streams join them at right angles (90°). Together they look like a garden trellis, a frame of crossing wooden strips. It forms on folded rock where hard and soft bands alternate. It matters because the pattern shows the rock structure hidden underneath. It is also one of the four patterns NCERT names (dendritic, radial, trellis, centripetal), so pairing questions are common.

Explanation

What decides any drainage pattern

A drainage pattern is the shape a river and its tributaries make on a map. (A tributary is a smaller stream that joins a bigger river.)
Three things shape it:
Slope: which way the land tilts and how steep it is.
Rock structure: whether the rock is the same everywhere, or has folds, faults, hard bands and soft bands.
History: how old the land is and what has happened to it, such as uplift or folding.
Over time a river cuts its valley along weak rock and weak lines. So the pattern on the map shows the hidden geology [2].

How a trellis pattern forms (cause → process → result)

Cause: folded rock with alternating bands
Earth movements squeeze rock layers into folds: long ridges and long valleys side by side.
Hard bands and soft bands alternate across the land.
Process: rivers follow the soft rock
Soft bands wear away faster than hard bands.
Rivers cut long valleys along the soft bands. So these streams run parallel to the ridges.
Short streams run straight down the sides of the hard ridges.
Result: right-angle joins
The short side streams meet the long valley rivers at right angles.
Long parallel streams plus short cross streams give a grid, like a trellis.
World setting: trellis drainage is a river's adjustment to tight regional folding in which the folds plunge (the folds tilt downward along their length) [2].

Parts of the pattern

Primary tributaries: long streams in the soft-rock valleys. They run parallel to each other and to the ridges.
Secondary tributaries: short streams that flow down the ridge slopes. They join the primary tributaries at about 90°.
Ridges of hard rock: the high, resistant bands between the parallel valleys.

Conditions it needs

Folded rock layers, not flat or uniform rock.
Alternating hard and soft bands, so erosion is uneven.
Ridges and valleys running in one main direction.
Without these, uniform rock gives a dendritic (tree-like) pattern instead.

Where it occurs

India
Folded Himalayas: rivers flow along the longitudinal valleys (long valleys running along the length of the ranges) between parallel ranges. Short streams come down the range slopes at right angles.
Singhbhum belt, Jharkhand: folded old rocks with hard and soft bands give a trellis pattern.
World
Regions of tight folding where the folds plunge [2].

Don't confuse with

Dendritic pattern: forms on uniform rock. Tributaries join at acute angles (less than 90°), like branches on a tree. Example: the northern plain rivers. Trellis forms on folded, alternating rock and has right-angle joins.
Rectangular pattern: streams also bend at right angles. But the cause is faults and joints (cracks in rock that cross at about 90°), not folds [2]. Examples are parts of the Vindhyas and the Chambal area.
Parallel pattern: many short streams run side by side down one steep, even slope, as with the short Western Ghats rivers. There are no right-angle secondary streams and no folded hard and soft bands.

Prelims Hooks

Trellis = parallel primary tributaries + secondary streams joining at right angles (90°).
Rock condition: folded rock with alternating hard and soft bands. Valley rivers follow the soft bands.
Indian examples: the folded Himalayas (longitudinal valleys) and the Singhbhum belt (Jharkhand).
Worldwide, it is linked to tight regional folding in which the folds plunge [2].
Trap: right-angle bends caused by faults and joints = rectangular (Vindhyas, Chambal), not trellis.
NCERT pair: dendritic vs trellis. The key difference is rock uniformity (uniform rock and acute angles vs folded alternating rock and right angles).

Mains Points

The pattern is a map of geology (GS-I)
A trellis pattern shows folded rock with alternating hard and soft bands, as in the folded Himalayas and Singhbhum.
Reading the pattern on maps and topographical sheets helps to map rock structure, folds and mineral belts. This matters most in Singhbhum, an old folded belt.
Planning in folded terrain (GS-III)
Rivers in trellis areas run along long valleys between ridges, and short, steep streams feed them from the sides.
The pattern affects how fast floodwater gathers. It also affects where dams, canals and interlinking projects can be placed, so it should guide water planning in the Himalayas.
Comparing patterns in answers: compare trellis (folds) with dendritic (uniform alluvium in the northern plain) and rectangular (jointed, faulted Vindhyan rock). This shows how one landscape factor, rock structure, produces different river shapes across India.

Related concepts

Drainage patterns

Sources

1Class 11, Ch 3 "Drainage System" (primary)
2River – Drainage patterns, Britannica — alsobritannica.com · tier 3