Floodwater from the Stillaguamish River surrounds a picnic table and trees.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Two invented 100 km² catchments. relief ×10. channel width symbolic.

Day 147 · rain-fed (gold) / snow-fed (blue). Snow store 80.0 mm water equivalent; snowmelt 17.9 mm/day.

How to read these results

Dam off. Annual regimes are invented and repeat after spin-up. They are not daily forecasts or records of named rivers.

River regime

Compare annual patterns of river discharge and explain how rainfall, snowmelt and dams can change their timing.

Drag to rotate. Hold Ctrl and scroll to zoom, or pinch on a touch screen.

Why it happens

A river regime describes the seasonal pattern of discharge. Rain may reach the river quickly, while snow stores water until melting; a reservoir can smooth and delay the resulting flow.

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The South Fork Stillaguamish River in flood. Water spreads beyond the usual channel when discharge exceeds channel capacity.

Try a comparison

Follow the melt season and compare its flow peak with winter rain. Switch the dam on and compare the peak and delayed release.

Reading the result

Both catchments use invented repeating annual inputs, and the dam represents storage behaviour rather than an actual operating rule.

A common misunderstanding

A river’s seasonal flow pattern depends only on the amount of rain each month.

Rainfall matters, but snow accumulation and melt, evaporation, groundwater storage and reservoir operation can shift the timing of discharge. Water received in one season may reach the river in another. Explaining a regime therefore requires both the inputs and the processes that delay or remove water.

Cases

Exam language

Regime

The characteristic seasonal pattern of river discharge, influenced by rainfall, snowmelt, storage and human regulation.

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USGS gauging station at Huey Creek, Antarctica, 12 December 2001. Repeated measurements of stage and discharge produce a river's hydrograph.

Nival

Describing a river regime strongly influenced by seasonal snowmelt. Discharge often increases during the thawing season. This differs from a glacial regime dominated by the melting of glacier ice.

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Seasonal snowmelt feeds a stream near Fourth of July Lake, Idaho. A nival river regime is strongly influenced by snow accumulation and melt; this photograph shows a snowmelt source, not a measured annual hydrograph.

Pluvial

Describing a river regime mainly controlled by rainfall; its discharge seasonality reflects rainfall and catchment storage.

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Floodwater on the Stillaguamish River, Washington. Rain-fed regimes respond to precipitation; a flow's competence is the largest sediment it can move, which cannot be inferred from water colour alone.

Snowmelt

Water released when accumulated snow changes from solid ice to liquid. Seasonal snowmelt can produce a strong spring or summer rise in river discharge. Its timing depends on temperature, radiation, elevation and snowpack depth.

Discharge

The volume of water passing a river cross-section per unit time, usually measured in m³ s⁻¹. For mean velocity v and cross-sectional area A, Q = Av.

Baseflow

The sustained part of river discharge supplied mainly by groundwater and other delayed stores between rainfall events.

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Groundwater emerges at a karst spring in Alabama. Water stored in connected pores and fractures can return to the surface as spring flow.

Dam regulation

Changes to a river's discharge timing and magnitude caused by storing and releasing water from a reservoir.

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Glen Canyon Dam and Lake Powell, Arizona. Reservoir storage and controlled releases alter the timing and size of flows downstream.

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