The catchment notebook

A river is a moving landscape.

Follow a river upstream and its story branches into smaller channels, hillsides, soils, and rainfall. Follow it downstream and the channel carries the combined effects of that catchment. This notebook looks at the shapes water makes and the measurements that help describe its movement without reducing it to a single line on a map.

Open the estimate a cross-section flow tool ↗

Study 04 / river / An original diagram

The workbench

Estimate a cross-section flow

Enter an average width, depth, and speed in metres and seconds. The product gives an approximate flow rate and its equivalent in litres per second. This simple section model helps explore relationships between dimensions. It does not replace a surveyed cross-section, a calibrated instrument, or a published gauge rating curve.

Three useful starting points

Observe · understand · practice

Start at the catchment edge

A catchment is the area draining toward a particular outlet. Its boundary often follows higher ground, but buried drainage and engineered channels can complicate the picture. Rain may run over the surface, soak into soil, or enter groundwater before reaching a stream. Hard surfaces usually change how quickly runoff arrives, while vegetation and storage can slow parts of the response. Trace small tributaries before drawing conclusions about a main channel. A river gauge sees what passes one location; it does not directly explain which hillslope supplied that water. Compare rainfall timing, soil wetness, and upstream storage when thinking about why two storms of similar size produce different river responses.

Meanders keep reshaping the bank

Flow around a bend often concentrates erosion near the outer bank and deposits sediment on the inner side. Over time the bend may migrate across its floodplain. Floodplains are part of the river system, storing and conveying water when flow spreads beyond the main channel. A neat-looking bank is not necessarily stable, and an old channel scar may still guide floodwater. Look for exposed roots, fresh sediment, and changes in vegetation from a safe public viewpoint. Avoid approaching undercut banks or entering fast water for measurements. A channel’s present shape records many past flows, not simply the water level visible during one quiet visit.

Read a gauge with its context

Stage is the water-surface height relative to a local reference point. Discharge is the volume passing a section each second. They are related through a site-specific rating curve, not by a universal conversion. Our estimator multiplies width, average depth, and average speed for a simplified rectangular section. A surface float usually moves differently from the depth-averaged flow, and real sections are uneven. Treat the result as a classroom estimate, never a safe-crossing decision. Compare units before multiplying: metres and metres per second yield cubic metres per second. A small rise in stage can accompany a large change in discharge when additional channel or floodplain area becomes active.

Take it further

A small next session

Sketch a cross-section from a photograph or a safe bridge viewpoint. Mark the banks, the water surface, and an imagined bed profile. Compare that shape with the rectangular assumption in the tool. Then halve the average speed while keeping width and depth fixed and explain the change in discharge. Write down which input is least certain. For real flood information, use the local gauge service and its warnings, including the timestamp and stated reference level.