Geomorphology

Geomorphology/Sediment Source Reports at SFSU

San Pedro Creek Longitudinal Profile – Fall 2001

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


What can we see in the profile?

Several things, for example:


  1. 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.
  2. 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.

Is there any hope? Can the creek be saved as a steelhead run?

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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