Stream Map Exercise: What Type of Stream Is It?
Step 1, Streams Present?
Find stream location on a USGS 7.5 Minute QuadrangleTopographic Map (1:24,000 scale). Locate a
stream channel or swale by finding V-shaped contour lines pointing upstream. Does the map represent
the stream channel with a dotted blue line (seasonal) or solid blue line (perennial)? Or is there a draw
with no lined stream (possibly ephemeral)?
Step 2, Ground Truthing Size:
Does the stream flow year round? Perennial streams flow at least 90% of the time. Is there a defined
channel indicating concentrated flow (seasonal streams)? If not, it is likely ephemeral, running only
during very wet conditions. Recall general stream width in feet, (measure when possible)
Step 3, Stream Order:
Is it a first order stream (a single small draw or dotted line with a very small watershed area? A second
order stream – you can see where two significant unlined draws or dotted line streams combine
upstream? A third order or larger will show combinations of larger watersheds of streams networks
upstream, and will likely be 12-25 ft in width, a significant creek.
Strahler stream order classification
Step 4: Calculate Slope
This is a rough guideline, as some small streams can be low gradient and vice versa, but it may give a
general indication of size and potential fish use. First, determine the contour interval (USGS Topo is
usually 40’ intervals, on the valley floor 10’ intervals will be dotted lines). Measure the length of stream
in inches. Convert inches to feet using scale bar on map (at 1:24K, 1”= 2,000’). Sum the elevation
difference using the contour intervals between the measured upstream and downstream points on the
stream.
Example: 3 contour lines crossed, vertical distance = 3 x 40’ =120’. Horizontal distance = 2 inches
(4,000’). Slope percent = Rise/Run = 120’/4,000’ x 100 = 0.03 x 100 = 3 % A 50 percent slope has a
vertical “rise” of 1 foot for every 2 feet of horizontal “run.” Such a slope is often called a 2-to-1 slope.
Don’t confuse slope percent with degrees. A 45 0 degree (100 %) slope is much steeper than a 50 %
(26.57 0) slope.
Step 4 slope
Step 3
Step 2 width & flow?
Step 1- blueline?
With the information gathered on Page 1, you can get a good idea of the size of your stream by seeing
where your answers fall in the table above. Low gradient streams would be most likely to be “Fish
Highways,” but Small and Medium streams may also be important fish habitat. Small streams are the
most likely to be classified as Type N, once surveys are made to determine fish presence.
Very Small
Unlined, but distinct Vshaped contours
pointing upstream,
indicating a draw or
swale
Small
Dashed blue line
Medium
Solid Blue line
Large
Solid Blue line
Ephemeral channels ,
springs, headwater
tributaries, etc., Often
lack defined channel
Seasonal tributaries
(usually unnamed), but
have defined channels.
Perennial streams
(usually Named)
Named Rivers and Creeks
ODF: Less than 2 cfs
avg annual flow Under
ODF: 2-10 cfs avg
annual flow
ODF: Over 10 cfs avg annual
flow
1st order,
under 200 ac watershed
area
7’ wide
1st or 2nd order.
Under 200 ac
watershed area
7-12’ width
2nd order or larger
Generally over 12’ wide
3rd order or larger
Generally high gradient
Often higher gradient
Lower gradient
20% or higher
Over 10% slope
Intermediate
gradient
3-12% slope
Often Rosgen Aa+
Often Rosgen A
Rosgen A, B or C
Rosgen B thru G
Under 3% slope