Mapping rivers & waterways

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For huge parts of the basin, rivers are the highways, the boundaries and the lifelines all at once. Accurate hydrology helps with navigation, flood planning and understanding how the landscape drains — which is why improving river geometry has always been at the heart of Amazon mapping.

How rivers are represented

OpenStreetMap describes flowing water in two complementary ways:

• The waterway centre-line — a single line running down the middle of the channel, in the direction the water flows. Every river needs this; it's what routing and analysis follow.
• The riverbank outline — for wide rivers, an area traced along both banks to show the channel's true width. Narrow streams need only the centre-line.

Smaller flows are tagged according to size, from broad rivers down to minor streams. Lakes, ponds and reservoirs are mapped as water areas. The small storage ponds that rural communities depend on — the kind volunteers traced during drought relief in semi-arid regions — are mapped as individual water bodies too.

Direction of flow

This trips up many beginners: a river line in OpenStreetMap has a direction, and it must point downstream. If you trace it the wrong way, you can reverse the line in the editor rather than redraw it. Where you're unsure which way the water moves, look for tributaries joining at shallow angles that point downstream, or follow the terrain from higher to lower ground.

Tracing a meandering channel

1. Follow the water, not a straight line. Amazon rivers loop and braid. Add nodes along each bend so the curve looks natural at full zoom.
2. Connect tributaries properly. Where a smaller stream joins a larger river, the two lines should share a node so the network is continuous.
3. Separate channel from floodplain. The permanent channel and the seasonally flooded land around it are different things — map the channel as water and, where relevant, note the floodplain separately.
4. Mind the season. Imagery taken at high water shows a far wider river than the dry-season channel. Map the normal channel and avoid baking a single flood moment into the permanent map.

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