Everything printed in blue on a USGS topographic map is water: streams, rivers, lakes, springs, canals, marshes, and even glaciers. The important part is the pattern. A solid blue line flows all year. A blue line broken by dots only runs part of the year. Dashed shorelines, dotted fills, and tufts of grass each tell you something different about whether you'll find water when you get there.
What does a dashed blue line mean on a topo map?
A blue line broken into dashes and dots is an intermittent stream: it carries water only part of the year, usually after snowmelt or heavy rain. A solid blue line is a perennial stream that normally flows year-round.
IN THIS GUIDE
Streams and rivers · Falls, rapids, and dams · Lakes, ponds, and washes · Springs, wells, and canals · Marshes, swamps, and flood-prone land · Finding water on a topo map · FAQ
Streams and rivers
Narrow streams are drawn as a single blue line. When a river is wide enough to show both banks at the map's scale, it becomes two lines with blue fill between them.
Perennial stream
A solid blue line. Flows all year in a normal year, so it's your most reliable water source. Even so, small perennial streams can shrink to a trickle in drought.
Perennial river
Two solid banks with blue fill. Wide enough that the map shows its real width. Plan on a bridge, ford, or boat to get across.
Intermittent stream
Blue dashes separated by dots. Runs only part of the year, typically spring snowmelt or the rainy season. Don't count on it in late summer or fall.
Intermittent river
Dashed banks with a dotted blue fill. A wide channel that is dry for part of the year, common in the arid West.
Disappearing stream
A stream that ends abruptly where the water sinks underground, often into limestone (karst) or loose desert gravel. It may reappear as a spring downhill.
Falls, rapids, and dams
These are the symbols paddlers watch for. Small versions are cross-marks on a single-line stream, and large versions are drawn to scale inside a double-line river.
Falls, small
A short bar across a single-line stream. On a narrow creek, it's worth scouting before you paddle through.
Falls, large
A waterfall drawn across the full width of a river. Expect a portage.
Rapids, small
Several short cross-bars on a single-line stream.
Rapids, large
Wavy lines filling a stretch of double-line river. The longer the patch, the longer the rapid.
Masonry dam
A concrete or stone dam drawn to scale, with the reservoir upstream and the stream continuing below it.
Dam with lock
A dam with a lock chamber so boats can pass, typical on large navigable rivers.
Dam carrying road
A dam with a road running across the top. Often the only crossing point for miles.
Lakes, ponds, and washes
Look at the shoreline. A solid outline means standing water. A dashed outline means the water comes and goes.
Perennial lake/pond
A solid shoreline with blue fill. Holds water year-round.
Intermittent lake/pond
A dashed shoreline with a dotted blue fill. Fills seasonally and may be a mudflat or meadow when you arrive.
Dry lake/pond
A dashed outline with a tan dot pattern, often labeled Dry Lake. These are usually desert playas that hold water only after rare storms.
Narrow wash
A dry streambed in arid country, drawn as a fine dashed line. Water flows only during storms.
Wide wash
A broad dry channel shown with a tan dot pattern or a dashed outline, often labeled Wash. Washes are easy walking routes, but never camp in one: flash floods arrive fast, even from storms miles away.
Springs, wells, and canals
The smallest blue symbols are often the most important in dry country.
Spring or seep
A small blue dot, sometimes with a short tail where the water flows away. Springs can be lifesavers on a desert route, but flow varies by season. Confirm recent reports and always treat the water.
Water well, geyser, fumarole, or mud pot
A small open blue circle, labeled with what it is. In Yellowstone and other volcanic areas, the same symbol marks geysers, fumaroles, and mud pots. Stay on boardwalks and trails.
Canal, flume, or aqueduct with lock
A blue line with a short cross-bar at each lock. Canals are man-made and usually run in straight lines or follow contours.
Elevated aqueduct, flume, or conduit
Water carried above ground on a raised structure.
Aqueduct tunnel
A dashed blue line where the aqueduct runs underground.
Marshes, swamps, and flood-prone land
Wetland symbols are small blue tufts of grass. What's underneath the tufts tells you how wet the ground is and whether there are trees.
Marsh or swamp
Blue grass tufts on white. Wet, soft ground, usually without trees. Slow going on foot, and buggy in summer.
Submerged marsh or swamp
Grass tufts over blue fill. Standing water with marsh plants: canoe country rather than hiking country.
Wooded marsh or swamp
Grass tufts over green fill. A forested wetland, which is what most people picture as a swamp.
Submerged wooded marsh or swamp
Tufts over a mix of green and blue: trees standing in open water, like a cypress swamp.
Land subject to inundation
A dashed blue outline, often labeled with a Max Pool elevation. Land that floods periodically, such as the upper end of a reservoir. Avoid camping here.
WATCH
Topo Map Reading, part 6: Map Symbols
Contour lines, benchmarks, rivers, streams, lakes, swamps, roads, trails, buildings, and campgrounds in under a minute.
Watch the full Topo Map Reading series →Finding water on a topo map
- Trust solid blue first. Perennial streams, lakes, and springs are the safest bets. Treat intermittent symbols as a bonus.
- Check the map date. Water features were mapped when the quad was made. Streams move, ponds silt in, and reservoirs get built. The date is printed in the lower margin.
- Read the contours too. Where contour lines bend into a V, the V points upstream. That's often where water collects, even when no blue line is printed.
- Look for the names. Names printed in blue italic belong to water features. A label like Coyote Spring or Tinaja marks a place where water has been found before.
PUT IT TO USE
See these symbols on a real map
Frequently asked questions
What's the difference between a solid and dashed blue line on a topo map?
Solid blue lines are perennial streams that normally flow all year. Blue lines broken by dots are intermittent streams that flow only part of the year.
What does the blue grass symbol mean on a topographic map?
Small blue tufts mark a marsh or swamp. On white, it's an open marsh. Over blue fill, there's standing water. Over green fill, it's a wooded swamp.
How is a spring shown on a USGS topo map?
A spring or seep is a small blue dot, sometimes with a short blue tail where water runs off, and is usually labeled with its name.
Why is a lake on my map dry?
It may be an intermittent or dry lake (dashed shoreline), or the map may predate a drought, a drained reservoir, or a natural change. Check the symbol and the map's revision date.
MORE SYMBOL GUIDES
Contour Lines on Topo Maps: Index, Intermediate & Depression Contours · Road, Trail & Railroad Symbols on Topo Maps · Benchmarks & Spot Elevations: What BM Means on a Topo Map
← Back to all USGS map symbols
Symbol artwork from the U.S. Geological Survey Topographic Map Symbols brochure (public domain).