CalWater Rules for Watershed Delineation
National Standards - Natural Resources Conservation Service (NRCS), Federal
Geographic Data Committee (FGDC)
The USDA
NRCS has worked with other Federal agencies to draft and adopt National
Watershed Delineation Standards. These have been reviewed and approved by the
FGDC as the National Standard for watershed delineation.
Watershed Delineation Rules Clarification
Watershed delineation will rely on the Federal Geographic Data Committee
(FGDC) Federal Standards for Delineation of Hydrologic Unit Boundaries (June,
2001) supplemented or clarified with the following rules:
- Existing 4th field HUCs may be changed only if they are not hydrologically
correct. To the extent possible, keep the existing HUC codes as they are.
- To the extent possible, delineate every true watershed as a distinct unit
at an appropriate level of the HUC hierarchy. For example, the North Fork Eel
River is 180,000 acres and is delineated appropriately at the 5th level. A
tributary to the North Fork Eel, Hulls Creek, is 50,000 acres and could also
be delineated at the 5th level. However it is not appropriate to break the
North Fork Eel into two 5th level watersheds (Hulls Creek and the remainder of
the North Fork Eel), rather Hulls Creek could be delineated as an oversized
6th level watershed. But Hulls Creek contains a tributary, Casoose Creek,
which is 1600 acres and also appropriate at the 6th level. In this case, the
North Fork Eel is delineated at the 5th level, Casoose Creek is delineated at
the 6th level, and Hulls Creek is not delineated at any level. Hulls Creek is
one of the exceptions to this rule because there is no way to accommodate each
pure watershed into the system.
- Maximize the number of true watersheds relative to the number of composite
watersheds. In any given larger unit (i.e. 4th level HUC), there should be a
nearly equal or larger number of true watersheds relative to composite
watersheds for the next smaller level (i.e. 5th level HUC).
- After delineating true watersheds, subdivide the remaining area into
suitable sized units based on tributary junctions. Delineate just upstream of
a confluence in most cases. Long-term major constructed features that have a
large impact on surface hydrology, such as dams or diversions, may be used
instead of tributary junctions to subdivide composite watersheds or split
oversized true watersheds into upper and lower reaches.
- Do not be concerned with an average size for a given level. A delineated
watershed is appropriate as long as it fits within its applicable size range
or is close to the size range upper or lower limits.
- Do not be overly concerned with composite watershed shape, unless the
shape is such that it appears to be two or more distinct watersheds to a
casual observer. There is a general tendency to split composite watersheds
more than necessary to gain a more roundish appearance. This is not necessary.
However there are cases where physical features would cause a composite
watershed to appear as distinct lobes separated by a narrow neck. It is
appropriate to split these composite watersheds at the neck to avoid confusing
users.
- Allow some latitude in acreage guidelines for special circumstances
involving true watersheds, but do not delineate hydrologic units smaller than
the next size smaller hydrologic unit. For example, a 45,000 acre watershed
splits relatively near the mouth into 20,000 acre Forks, leaving a remaining
area of 5,000 acres. This remaining area would normally be considered too
small to delineate at the 6th level, but do so anyway to avoid compromising
the true watershed integrity of one of the Forks. But if the Forks were each
22,000 acres, leaving a 1000 acre remnant, include the remnant with the larger
Fork as it is smaller than 2,500 acres, the small limit to what would normally
be delineated at the 7th level.
- Delineate watersheds based on surface topographic features, where surface
water would flow naturally, primarily natural features but may include
constructed features is special circumstances. In difficult areas such as lava
flows (areas with little expression of surface water flow), delineate based on
topographic expression that would determine the flow of water if rainfall were
heavy enough to cause surface flow. Constructed features may if used to
determine watershed boundaries if the feature controls all surface flow all
the time the same as a topographic watershed divide, as occasionally happens
with canals, ditches, or storm drains in some locations. The flow-controlling
feature such as the downhill levy of a cross-contour ditch represents the
watershed divide.
- Hydrologic units must be contiguous in nearly all cases, in other words it
is generally inappropriate for two distinct, unconnected polygons to contain
the same Hydrologic Unit Code. The only exception to this rule may occur in
frontal (ocean or lake front) watersheds where a small section of shoreline is
isolated by a pure watershed. For example, there is a narrow strip of beach
front between the mouths of the Klamath River and Redwood Creek, north of
Redwood Creek but within that same 4th level HUC. It is much too small to be
delineated at the 5th level although Redwood Creek is delineated at the 5th
level. This frontal strip is combined with the frontal area south of Redwood
Creek to form a 5th level frontal watershed even though the two areas are
distinct polygons separated at the mouth of Redwood Creek. But this
arrangement, two distinct polygons forming one HUC, is more appropriate than
the alternative that would be to compromise the pure watershed integrity of
Redwood Creek.
- Bays, inlets, and estuaries may be included with either the connected
water watershed or the adjacent land watershed, depending on circumstances. In
general, if a bay contains a narrow connection to its adjacent water body
(relative to the size of the bay) that can be considered a pour point, then
the bay is delineated as part of the land hydrologic unit. If a bay does not
contain a distinct pour point, it is considered part of the water body. For
example, the San Francisco Bay is considered part of a San Francisco Bay
hydrologic unit, with its pour point at the Golden Gate, while Monterey Bay is
considered part of the Pacific Ocean.
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