Watch rainfall turn into runoff and carve a hillslope in real time. This simulation models sheet flow, channel concentration, and sediment transport using a simplified stream-power erosion law, so you can see directly how rainfall, slope steepness, vegetation cover and soil type interact to produce anything from a stable, grassy hillside to a deeply gullied badland.
Hydraulic erosion follows the stream power law: erosion rate scales with slope steepness and accumulated runoff raised to a power greater than one, which is why erosion concentrates sharply into channels rather than spreading evenly across a slope.
Increase rainfall and reduce vegetation cover to watch gullies carve into the slope within seconds. Switch soil type to compare how sand, loam and clay balance infiltration against erodibility, and use Reset to start from a fresh, undisturbed profile.
Vegetation is one of the most effective erosion controls known: root networks can cut soil loss by more than 90% compared to bare ground on the same slope, which is why reforestation and cover cropping are core strategies against desertification.
This simulation renders a hillslope as a one-dimensional heightfield and drives it with a simplified stream-power erosion model, the same family of equations geomorphologists use to explain bedrock river incision and hillslope gully development. Each simulated column receives rainfall, loses some of it to infiltration depending on soil type, and passes the remainder downhill as accumulating runoff. Wherever the product of local slope steepness and accumulated flow is large, the model detaches soil mass at a rate proportional to that product raised to roughly the 1.5 power — a nonlinear relationship that explains why erosion concentrates sharply into narrow channels rather than lowering a slope evenly.
Detached material becomes suspended sediment that is carried downhill until the flow's transport capacity falls below the sediment load, at which point it is deposited again, building alluvial fans and terraces lower on the slope. Vegetation cover suppresses erosion multiplicatively by binding soil with root networks and slowing overland flow, while soil type trades off infiltration capacity against erodibility: sandy soils absorb rain quickly but resist very little once water is moving, while clay sheds most rainfall as runoff yet holds together better against detachment. Adjusting rainfall, slope angle, vegetation and soil type together lets you explore why some landscapes stay stable for centuries while others gully out in a single storm season.
What does this simulation show?
It models rainfall runoff eroding a hillslope cross-section in real time, using a simplified stream-power law where erosion depends on both the local ground slope and the amount of water flowing over each point. Over time you can watch stable slopes stay smooth while unprotected, steep or heavily rained-on slopes carve themselves into gullies.
What is the stream power law?
It is a widely used erosion model stating that erosion rate scales with slope steepness multiplied by drainage area (accumulated flow), raised to a power typically between 1 and 2. Because the relationship is nonlinear, small increases in slope or flow can produce disproportionately large jumps in erosion — which is exactly why water tends to carve narrow, deep channels rather than eroding a hillside evenly.
Why does vegetation reduce erosion so much?
Plant roots physically bind soil particles together and increase the soil's resistance to being dislodged, while stems, leaves and leaf litter slow down overland water flow and encourage infiltration instead of runoff. Research on vegetated versus bare slopes commonly finds erosion reduced by well over 90 percent once a healthy plant cover is established.
Rainfall intensity sets how much water lands on the slope each tick; slope angle sets the steepness and regenerates the terrain profile; vegetation cover scales down the erosion rate to represent root binding and canopy protection; the soil type selector switches between sand, loam and clay, each with different infiltration and erodibility constants; and Reset restores a fresh, lightly undulating slope with the erosion tally zeroed.
Sandy soils have large pore spaces so water infiltrates quickly, leaving less to run off, but individual sand grains bind together weakly so once water is flowing it detaches them easily. Clay particles are extremely fine and bind tightly together, resisting detachment, but clay's tiny pores infiltrate water slowly, so most rainfall becomes runoff. Loam sits between the two, which is one reason it is prized for agriculture.
It is a running total of all the soil mass the model has removed from the slope since the terrain was last reset, summed across every column and every simulated tick. It rises fastest when rainfall is high, vegetation is low, the slope is steep and the soil is erodible, and it gives you a single number to compare different settings against each other.
Flowing water can only carry a limited amount of sediment at once, called its transport capacity, which depends on how much water is flowing and how steep the local slope is. When eroded material exceeds that capacity — for example where the slope flattens out — the excess sediment is deposited again, building up material further down the profile rather than disappearing.
It captures the correct qualitative behaviour and the core nonlinear stream-power relationship used in real geomorphology research, but it is a simplified one-dimensional cross-section rather than a full two-dimensional hydrological model. Real landscapes also involve groundwater flow, soil layering and weathering processes that this simulation does not attempt to reproduce.
Runoff accumulates as it moves downhill, since every column adds its own rainfall contribution to the flow already arriving from upslope. Because erosion in the stream-power model depends on that accumulated flow, points further down the slope typically experience both more water and, if the slope is well developed, similar or steeper local gradients — so erosion is rarely uniform and tends to concentrate downstream.
Stream-power erosion models underpin flood-risk mapping, agricultural soil conservation planning, river channel engineering, and landscape evolution studies used to reconstruct how mountain ranges and valley networks formed over geological time. The same principles guide practical decisions like contour ploughing, terracing and reforestation to protect farmland from runoff damage.