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A
longitudinal profile is a graph that stream scientists –
such as fluvial geomorphologists – use to study the
gradient of creeks and rivers at various locations along
its path from its headwaters to its mouth. (For our profile,
we stopped at Peralta Bridge because the Flood Control Project
a short ways downstream will dramatically change the profile
in that reach, as soon as it opens.) The graph "straightens
out the curves" of the creek displaying channel length
on the horizontal axis and channel elevation on the vertical.
In order to see the whole profile, the view shown here has
a vertical exaggeration of 43 times.
For
the survey, the team used a tripod-mounted optical level,
stadia rod and survey tape to capture elevations of (1)
the deepest part of the channel – called the thalweg
(Ger. "valley way"); (2) the water surface at
base flow; (3) the bankfull elevation; and (4) various terraces.
This technique was very accurate: in a 8200 feet length
of the channel – from Peralta to Linda Mar Blvd. bridges
– the elevation error was only 0.01 foot.
San
Pedro Creek Longitudinal Profile
Click
image to enlarge
Several
things, for example:
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The creek's profile has some peculiar patterns that relate
to the history of impacts. Clearly the biggest impact
can be seen at Capistrano Bridge, where a knickpoint
was created in the early 1950's by a grade control structure.
Since that time, fifteen feet of vertical erosion downstream
of the structure has created a serious barrier to fish
migration, plus some serious headaches for downstream
residents losing their backyards. A succession of largely
ineffective fish ladders have been installed, but this
barrier remains and addressing it is a top priority for
improving passage to important habitat upstream.
Both
downstream and upstream of Capistrano, the creek's gradient
has eroded to a lower gradient than must have existed
before settlement. The average baseflow gradient below
the Capistrano fish ladders and above Adobe Bridge is
0.91%, with a similar gradient (0.90%) at the bankfull
level. In this reach, the uppermost terrace, which appears
to relate to the bankfull level before settlement, has
a gradient of 1.07%. The lowered channel gradient is likely
the result of erosion from more frequent peak flows from
urban runoff (rainfall on paved areas runs off quickly.)
We are also seeing the effects of past channelization
downstream, creating a headcut which has migrated up to
Capistrano bridge. The potential for further erosion will
depend upon whether this profile is flat enough to be
in dynamic equilibrium with the flashy urban runoff it
is provided. Unfortunately, the likelihood is that it
is not, and even more erosion will occur unless something
is done to decrease the flashiness of the runoff.
Most
of the bridges serve as grade control structures, and
this can be seen by looking at the profile at these points.
Bridges at Adobe, Capistrano, Linda Mar and Oddstad all
force the creek through concrete box culverts, creating
a limit to downward erosion at that point in the profile.
This is ok for the sections immediately upstream, but
it invariably creates a fish migration barrier as a steep
step and deep pool develops downstream. This is why people
often see fish on the downstream side of Adobe Bridge.
Below
the North Fork confluence and extending downstream to
the next grade control structure at Linda Mar Bridge,
the gradient has similarly been flattened as a result
of urban runoff, primarily from the North Fork watershed
and delivered by its system of culverts draining Park
Pacifica. While the upper terrace gradient is 1.85% in
this reach, the water surface and bankfull gradients are
1.09% and 1.07% respectively.
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For
the same reason, the main-stem upstream of the North-Fork
confluence has been steepened to a gradient of 1.8%. This
is because the downcutting below the confluence creates
a steeper gradient in the main channel draining into it.
This steeper gradient will no doubt create a headcut,
which will migrate upstream until it reaches the next
grade control structure at Oddstad Bridge. This is where
we should expect the next major barrier to fish migration,
as a deep pool develops downstream of the concrete pad
under the bridge.
There is some hope. One, the flood control project in the
lower reaches may help to prevent new headcut cycles from
initiating, though only in the lower sections -- it doesn't
of course remove the effect of flashy urban runoff and the
erosion that it creates. Two, by knowing where erosion is
likely to create barriers, we can start to address the problems
before they get out of hand. Three, we can use this information
to identify good areas for restoration, and to plan those
projects. Clearly, the barrier at Capistrano Bridge is a
top priority; in fact, the city is under directive by the
National Marine Fisheries Service to remove this barrier.
But there are many other areas needing attention.
Repeating
this survey in a few years will also help us to understand
erosion rates in various reaches. The section from the park
down to just below the North-Fork confluence has in fact
been surveyed on three successive years recent, and from
this temporal view we can see in this some surprising effects.
For example, upstream of the Oddstad bridge grade control
structure, there has been significant downcutting – approximately 1.5 feet in two years. Why is this happening
in a park, with the only significant rapid runoff coming
from the few gravel roads and a small parking lot at the
visitor center? The answer, being investigated by students
at SFSU, appears to be that we're seeing the response of
the creek to a major depositional event from the
1962 debris flow that wiped out John Gay's trout farm operation.
The creek is now rapidly cutting through these quite recent
deposits.
We
will probably discover other surprises when we repeat the
downstream survey. We may be able to detect which sections
are possibly stable, and which are heading for failure.
We will need the cooperation of all creekside residents.
The key to the success of any restoration project is taking
the longitudinal view: (a) what happens at any
point relates to things both upstream and downstream; and
(b) what you do to any part affects the creek both upstream
and downstream. This profile helps us to see it.
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