Calibrating Bank Full Measurements Using Regional Curves and USGS Stream Guage Data

Bankfull is important to fluvial hydrogeomorphology (HGM) because it often determines the shape of the channel by moving and depositing sediment. Bankfull (BF) is defined as the high water level that recurs every 1 - 2 years, but measuring it in the field involves using multiple indicators in a 'preponderance of evidence' detective-style approach.

Most plants that cannot tolerate saturated soil conditions for days at a time, like Alders, will not grow below BF, while willows and cottonwood can. Also, the top of point or side bars can indicate the height of BF, but on the Rio Embudo, near Dixon NM, BF indicators were contradictory and hard to find. Is BF just a few centimeters above the base-flow water, or are all the willow below BF?

From Rio Embudo at Dixon, NM Hydrology Analysis

A number of bars and scour features at different heights further compounded the mystery. It was time to seek out other clues. One source of potential indicators was our aerial imagery, which was taken during Spring runoff, 2008:

From Rio Embudo at Dixon, NM Hydrology Analysis

The point bars at bottom right are bisected by a side channel that is several feet above the base level today. That means BF must be at least that high, and would probably inundate most of the willows. Corroborating this, the landowner reports that the willows are indeed flooded almost every year. But exactly how high is BF? To gather more data, we surveyed three channel cross sections, or transects (TR), noting the heights of the major terraces.

TR-Upper

From Rio Embudo at Dixon, NM Hydrology Analysis

TR-Middle

From Rio Embudo at Dixon, NM Hydrology Analysis

Tr-Lower

From Rio Embudo at Dixon, NM Hydrology Analysis

On each of these cross sections we marked where the current base flow water level is, where we think BF is, and where we think Flood Prone (FP) might be. To check these guesses, we correlated those heights with flow data from a USGS gauge just downstream:

From Rio Embudo at Dixon, NM Hydrology Analysis

From this graph we could see that the high water level with recurrence every 1 -2 years is about 400 cubic feet per second (CFS). We could also see that the current flow was about 38 CFS. If the Rio Embudo is flowing with 38 CFS today, how high would a BF flow of 400 CFS be?

between the flow today and BF flow. To figure that out we might need to correct for any changes in the velocity (feet/second). Manning's Equation:

shows that velocity V is proportional to a constant, u, inversely proportional to a coefficient of friction, n, varies to the 2/3 power of channel cross-sectional area, R, and to the 1/2 power of slope, S. Since neither slope nor the constant would change, we can discount them and focus on n and R; n will likely increase because the willows will act like a series of giant combs, increasing friction, and R will also obviously have to increase. For example, doubling the height of the water would multiply that term by 1.6. Unfortunately, coefficients of friction need to be experimentally determined, so we can only guess at n. To make things easier, I decided friction would also increase by a factor of 1.6, to exactly cancel out R. In other words, I don't think the velocity would change by much.

So it is a simple matter of geometry to calculate the cross-sectional area that would correspond to 400 CFS on our cross sections (red lines on the cross-sections, above). Without exception, this height is higher than our field-determined BF (green lines on the cross-sections, above) and, at least for TR-L, even higher than our FP height.

But is this right? Are we getting closer to the truth? To check, we can calibrate our answers for the Rio Embudo against data published by Natural Channel Design on a large number of other Southwestern rivers:

From Rio Embudo at Dixon, NM Hydrology Analysis

I plotted both our field-determined BF cross-sectional area (green points) and the USGS-determined BF cross-sectional area (red points) on the regional curve above. The green points seem to fall on the line for New Mexico, while the red points fall on the Arizona line, corroborating our field measurements and casting doubt on the USGS. However, the watershed above Dixon is very impermeable and could behave more like AZ than NM. I think the true value is probably somewhere in-between the field and USGS values.

This line is probably as close as any to Bankfull:

From Rio Embudo at Dixon, NM Hydrology Analysis

Evaluating Restoration Potential at Taos-area Streams

We use a four-tiered scale to rate restoration potential, basically A, B, C, D:

A is fine, possibly excepting some irrigation
B is for "needs beaver!", and/or stop grazing
C needs instream structures and earth moving equipment to restore functionality
D can't be helped

From Natural Heritage New Mexico - Taos Streams

Rio Costillo. Upstream landowner's home is within flood-prone area and would certainly be an impediment to any restoration effort, while downstream landowner's stretch has been dredged. D

From Natural Heritage New Mexico - Taos Streams

Rio Fernando: A thriving beaver population has created extensive wetlands composed of Typha and Salix exigua, but completely extirpated Populus from the reach. A (maybe plant cottonwood)

From Natural Heritage New Mexico - Taos Streams

Santa Barbara Upper: A main channel circumvents the beaver ponds and side channels and appears somewhat channelized, with low habitat diversity (and hence few fish). What is causing the main channel to bypass beaver dams and downcut? A?

From Natural Heritage New Mexico - Taos Streams

Rio Pueblo: Irrigation returns from irrigated pastures along the North; humans have attempted to replace breached beaver dams and drying beaver ponds with rock-and-plastic "fisherman's dams". B

For more site descriptions, click on the links above.

Rio Puerco, NM: BEFORE and AFTER

The furthest downstream restoration site is doing well. Note how the roads have revegetated.

From Rio Puerco Restoration Evaluation - September 2009

The large berm in the background is the "remeander" dam that was built a couple years ago to push the Puerco back into its original channel. In the foreground an In-Stream Willow Planting (ISWP) immediately after it was planted (left) and then after a single growing/flooding season (right). This planting did not incorporate wattles. It has largely washed out as the river channel continued its lateral migration toward river right. A large amount of new erosion is visible along the right side of the channel, which is exactly what this planting tried to prevent. Unfortunately, it is working against the natural migration of the river.

From Rio Puerco Restoration Evaluation - September 2009

The same ISWP, looking upstream. The main channel is now flowing (and cutting) right along the bank. The point bar on the left has widened.

From Rio Puerco Restoration Evaluation - September 2009

These photos show the Guardian's plantings immediately downstream from the bridge. This old channel is now receiving flow because of the remeander. It is a healthy channel with an active floodplain. All of the cottonwoods and willows received overbank flooding from the summer monsoons, and did very well. Its a real jungle down there!

From Rio Puerco Restoration Evaluation - September 2009

These photos show an ISWP on Guardian's State Land. It is the middle of three such ISWPs on State Land. The channel through this reach is downcut and straight, almost like a ditch, and the plantings sought to induce meanders. While the plantings didn't wash out, it doesn't look like the willow are doing very well; in this wet year the bottom of the channel may have been too wet for them. Looking further downstream, one can see sedges growing into the channel, forming a natural meander pattern. Planting sedge plugs may be more effective below bankfull.

From Rio Puerco Restoration Evaluation - September 2009

These photos show the downstream area of the Guardian's State land that was planted in 2008. The ISWP from 2008 is not quite visible under the large cut bank in the center distance. The development and change of the river's channel and associated point bars are visible in the foreground.

From Rio Puerco Restoration Evaluation - September 2009

For more information and more photos, click on one of the photo links above.

Happy Equinox

Today the day and the night are 12 hours each, equal, everywhere in the temperate regions (everywhere that's not the tropics). Since the last, Spring, Equinox, the days have been longer the farther North you are, although they have been longer than the nights everywhere North of the Tropics. But after today farther North places will have shorter and shorter days, while the farther South you go the closer you get to a perfect 12 hour day and 12 hour night. The Equinox also marks the time of the year when the length of days is changing fastest (the second derivative is greatest). This is the time of year when people look up and remark "Fall is coming!".

What are the Laws of Ecology?

PROVISIONAL: (thanks to ecolog subscriber Dr. KC Jensen)

1. There is No Such Thing as a Free Lunch

2. Mother Mature Knows Best

3. Everything Has to Go Somewhere

4. Everything is Connected to Everything Else

New Mexico Heritage Field Work

From New Mexico Work

drying up beaverpond

From New Mexico Work

From New Mexico Work

From New Mexico Work

Eastern US Trees with Disjunct Populations in the SW's Mountains

Cherries
Hophornbeam (Ostrya)
Mulberry
Dogwood
Acer negundo
Rhus glabra
Soapberry Sapindus
Prunes (Prunus)
Gum Bumelia
Sycamore (Platinus)
Salix nigra....
Ash

Oak?