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go to chapters:
1. Overview
2. Study Area Description
3. Geomorphology and Sediment Cones
4. Environmental Resources
5.Problems and Opportunities
6. Recommendations
7. References
8. Appendices
CHAPTER 3 GEOMORPHOLOGY AND SEDIMENT CONES
3.1 Hydrology
3.2 River Channel Geomorphology and Cross Section Changes
3.3 Sedimentation Observations within the Forgotten River Reach
3.3.1 Limitations
3.3.2 Introduction
3.3.3 GIS Assessment
3.1 Hydrology There are currently three stream gages in the project reach, located at Fort Quitman, Candelaria, and just above Presidio and the confluence of the Rio Conchos. These gages are operated by the International Boundary and Water Commission. Click for flow information:
Figure 2 - Fort Quitman IBWC gage in the rectified channel at the upper end of the Forgotten River Reach looking downstream
Figure 3 - Candelaria IBWC Gage upstream of Capote Creek and Candelaria, Texas, January 2007. Note aggradation towards the top of the image from influence of San Antonio Diversion Dam (Mexican) just downstream and San Antonio arroyo confluence just upstream.
(return to table of contents)
The first ditch for irrigation in the study area is reported to have been at Ruidosa in 1872. Other small ditches were constructed in the next decades. By 1900, farming had been extended northward to Pilares. Examination of aerial photographs made in 1928 show that roughly 50 percent of the floodplain from the Rio Conchos to Candelaria was under cultivation, but upstream, there was very little agricultural activity. On the U.S. side the irrigated area increased to a peak of 10,000 acres during 1928-1930 and then declined.
Prior to the advent of intensive irrigation and to construction of the Rio Grande Project (most importantly Elephant Butte and Caballo Dams and Reservoirs, but also Percha Dam, Leasburg Dam, Mesilla Dam, American Dam, International Dam, and Riverside Dam), the Rio Grande below El Paso generally experienced biannual seasonal flows. From April through June, snowmelt runoff from southern Colorado and northern New Mexico typically delivered a majority of the annual flow. In the summer monsoon months of July to September, flash floods from tributary arroyos provided substantial flows into the main river channel. During the 1800s and early 1900s, the river was indeed considered navigable for 100-150 ton keelboats from a point approximately 147 miles below El Paso (Kelley 1986). Before Elephant Butte Dam construction, El Paso and Juarez farmers witnessed huge annual flows above one million acre feet in 1891, 1897, 1903, 1905, 1906, 1907, 1911, 1912, and 1914; and only trickles of less than 100,000 acre-feet during 1894-96, 1899, and 1902 (IBWC 1978). Consequently, farming communities affected by these unreliable flow conditions clamored for the U.S. government to dam and control the river.
Since the construction of the Elephant Butte Dam in 1915, the character of the Rio Grande has been dramatically altered. However, irrigation withdrawals in the El Paso/Juarez valley were sufficiently extensive prior to 1915 that the magnitude and duration of the annual snowmelt flood had decreased by more than half between El Paso and the Rio Conchos (Schmidt et al. 2003). The magnitude of the 2-year recurrence flood, decreased from 7380 feet/second prior to 1915 to 4308 feet/second post-dam between El Paso/Juarez and the Rio Conchos (Table 1). In those years, when the annual peak flow at El Paso/Juarez was less than 3531 feet/second, no snowmelt flood peak reached the Rio Conchos. In years of greater snowmelt runoff, the magnitude of the peak flow at the Rio Conchos was never more than 90% of that measured at El Paso/Juarez, and typically occurred 7-10 days after the peak had passed El Paso/Juarez.. The only times when stream flows at Presidio were significantly larger than at El Paso/Juarez were in the late summer and early fall when flood flows were triggered by rainfall in the downstream tributaries of the basin.
Table 1 - Magnitude of floods of different recurrences at El Paso and upstream of the Rio Conchos
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Discharge in cubic feet per second, of annual maximum mean daily discharge, for indicated period at indicated location
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Prior to 1915
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1.25 year
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2 year
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5 year
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10 year
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At El Paso
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3460
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7380
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13347
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17090
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Above Rio Conchos
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1836
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4308
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8616
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16652
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1915 – present
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1.25 year
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2 year
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5 year
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10 year
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At El Paso
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1130
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1801
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3531
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4378
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Above Rio Conchos
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530
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1201
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2472
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349
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Presently, spring runoffs from snowmelt in the upper Rio Grande Basin and the flash flood swells in summer months have been impounded, impeded, and controlled. The energy within the Rio Grande, i.e., peak flows, and the ability to transport sediment, has been significantly reduced. The river between El Paso and Presidio is now an aggrading reach of stream whose bed is substantially higher than prior to dam construction upstream (Schmidt et al. 2003) The impacts of Caballo Dam and downstream diversion dams have compounded the shift from a seasonally meandering wild river to a regulated irrigation project generally confined within constructed levees. Effects of these upstream impacts have had consequences, usually adverse, in the Forgotten River Reach. Over the years, the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) constructed 12 flood and sediment detention dams on tributary arroyos to the Rio Grande between Caballo Dam and the head of the study reach. These structures regulate flows from 533 square miles out of a direct contributing area of 1,650 sq. miles between Caballo Dam and the study area and serve to reduce sediment contributions from these arroyos and temper peak flows. Today, the biannual peak flows in spring and summer have been replaced by a low, steady flow regime tied to the irrigation season. Typically, annual irrigation releases begin in February and last through October (Landis 2001).
Ultimately, the volume of flows through the Forgotten River Reach of the Rio Grande post 1915 is only one quarter of the annual quantity present prior to the construction of Elephant Butte Dam (Landis 2001) and the 1944 Treaty between the United States and Mexico which specified delivery from the Rio Grande of 60,000 acre feet of water annually to Mexico through the Acequia Madre canal just below El Paso.
There are 101 arroyos that provide flash flood runoff into the Rio Grande in the Forgotten River Reach, which originate in the bordering mountains on the U.S. (57) and Mexican (44) sides and which deliver large quantities of heavy sediments into the river channel. Because of the river’s flatter gradient, much of the tributary sediment deposits into the river channel. These deposits have caused the channel cross section to progressively decrease in size. In some places, such as upstream of Indian Hot Springs on the Rancho Consuelo, it is now only three feet or less in width and one foot or less in depth. Nearly 30 years ago, the channel was, in some places, barely discernable due to its becoming filled with sediment and overgrown with salt cedar (IBWC 1978).
Recently, a photojournalist who boated the entire Forgotten River Reach stated how often he followed what he thought was the main channel only to reach a dead end thicket of salt cedar, thus to work back upstream to find the true channel (Nat Stone, pers. comm. 2007). Fifty-six of these arroyos showed evidence of depositional activity at their confluence with the main stem, based on photogrammetric interpretation by University of Texas Center for Space Research (UTCSR) in 2006. This activity varied from small but significant depositions that affects existing vegetation to major depositions that have visibly pushed the channel compared to previous years’ photography (e.g., Green River confluence [Figure 3] San Antonio Arroyo confluence). Unfortunately, ground truthing of many of the 56 sites evidencing some level of depositional activity through photogrammetric interpretation was not possible by Corps personnel due to private lands access issues. (return to table of contents)
Figure 4 - Confluence of the Green River with the Rio Grande (42 river miles downstream of Indian Hot Springs, Texas). Note the sediment plug. Photo February 2007 (return to table of contents)
Figure 5 - Upstream view of the Rio Grande and Green River confluence showing relative elevation of sediment plug and abrupt change in gradient of the Rio Grande; looking upstream. Photo February 2007 (return to table of contents)
Data assimilation and photogrammetry by UTCSR indicates 54 levee structures within the project study reach (see map for locations), only three of which are clearly owned by the IBWC. The remainder appears to be levees built over time by various entities to protect agricultural lands and to influence the course of the river. Most of these non-IBWC levees bear little relationship to the current location of the channel or floodplain. Nearly all are overgrown by trees and brush. Additionally, the UTCSR geospatial database shows numerous (13) drains owned by the IBWC, and one rip rap grade control structure.
Flows The flood peaks are soon dissipated after they enter the river, by overbanking and floodplain and channel bank storage (examples of attenuation can be seen on aerial imagery where wide expanses of salt cedar occur, e.g., upstream of Indian Hot Springs) . This phenomenon is reflected by the records of flood peaks at the Presidio gage near the downstream end of the reach above Rio Conchos, which shows that during the period of 1951-76, floods exceeding 500 cubic feet per second (cfs) occur only about once a year on the average and peaks over 2,000 cfs only about once every 15 years on the average (IBWC 1978). However, from a total annual volume perspective, almost 93% of the flows recorded at Fort Quitman arrive at the Presidio gage. Apparently, the in-stream losses due to evapo-transpiration and seepage are nearly offset by the summer rains which fall below feet Quitman. The annual hydrograph for the three gages in the Forgotten River Reach indicate that in some years the reach is a gaining river (Landis 2001)
Figure 6 - Extreme drought and high flow years as recorded at the upper end of the study area, at the Fort Quitman gage
The timing of peak flow months has shifted by an average of three months later in the season as a result of storing and releasing water for irrigation purposes. Prior to 1915, the average monthly volume of water in El Paso averaged nearly 220,000 acre-feet in the month of May. After the completion of Elephant Butte Dam the peak month shifted to July, with a maximum average volume of 63,700 acre-feet for the last 60 years. Similarly, the peak flow at Presidio prior to 1915 occurred in July with an average of just under 160,000 acre-feet, and during the last 60-year period, the peak month has been October with an average volume of 17,700 acre feet, a reduction in peak flow of almost 90% (Landis 2001). Thus the peak period of spring runoff flow has been delayed by three months for the Forgotten River, and markedly diminished in quantity. The figure below depicts these changes in maximum average monthly volume, but not the temporal delay associated with them. (return to table of contents)
Similarly, prior to 1915, the annual average amount of water reaching Presidio equaled 573,700 acre-feet. Since the completion of the Elephant Butte Dam, the total annual volume of water has been reduced by 77% to 131,800 acre-feet. This change in flow patterns through the Forgotten River has critically reduced the river’s capacity for sediment transport. Additionally, the resulting river system now experiences a higher proportion of arroyo contributions into the total flow through the Forgotten River. These monsoonal flood events deliver huge amounts of sediment into the main river channel, a river now lacking the velocity and quantity of flow to effectively transport these loads. Consequently, enormous sediment bars reside at the mouths of the arroyos which previously were transported by the river for dispersion in downstream floodplains. Today, there are sections of the Forgotten River which are aggraded, having a river bed higher than the surrounding floodplain (Landis 2001).
Extensive sheet erosion and gullying often results from high velocity flood waters flowing over the steep gradient mountain fronts and heavy bank erosion occurs in arroyos. Much of the arroyo sediments are deposited in arroyo deltas in the Rio Grande floodplain and channel. The average annual sediment deposition in the Rio Grande channel is estimated to be in the order of about 300,000 cubic yards per year, and on the floodplain in the order of 500,000 cubic yards per year. The arroyo flows include silts, sands, and gravels, the larger particles being deposited near the arroyo mouths. (return to table of contents)
3.2 River Channel Geomorphology and Cross Section Changes
The U.S. and Mexico under Convention of February 1, 1933, stabilized the international river boundary in the El Paso-Juarez Valley and provided flood protection to this area by rectifying a reach of 155 miles of the formerly meandering Rio Grande and shortening it to a distance of 86 miles. This action had considerable effects upon the geomorphology and hydrology and hydraulics occurring in the Forgotten River Reach.
The Rio Grande in the study reach follows a sinuous channel a distance of 198.8 miles between a point about 13 miles downstream from feet Quitman, Hudspeth County , and a point near Haciendita Ranch, about 6 miles upstream from Presidio, Presidio County The straight line distance through the reach is 117 miles.
Upland plains have slopes of about 60-200 feet per mile to the Rio Grande floodplain. In contrast, the Rio Grande has an average gradient of 4.5 feet per mile through the study reach (Fullerton and Batts 2003).
Elephant Butte Reservoir has controlled the snowmelt floods which prior thereto flushed sediment through the channel in the Study reach. However, tributaries to the Rio Grande below Fort Quitman have continually deposited sediment into the study reach and, absent spring runoff flows; progressive aggradation of the channel has resulted.
Since construction of Elephant Butte Dam, the Rio Grande channel form has changed from wide and moderately deep to a shallower channel with a marked decrease in cross-sectional area. The decrease in cross sectional area has been more pronounced since 1950, incident to the decreasing flows. This is clearly demonstrated by a comparison of photographs taken some 40 to 60 years ago and in 1977. Because of the decreased channel size, the frequency of overbank flooding due to arroyo floods has increased. The decrease in channel cross sections is further demonstrated by surveys made from time to time of the Rio Grande channel in the upper, middle, and lower parts of the Study reach. The first cross section surveys were made in 1935-36 and continued through 1977. The data show the channel was progressively decreasing in size throughout the study reach. The data indicated that for the upper segment of the study area, cross sectional area in 1977 was 22% smaller on average of what it was in 1935; in the middle segment, 17% of the 1935 area; and in the lower segment, 8% of the 1935 area. This data demonstrated that, even in 1977, it was only a matter of time until there was no identifiable channel, a condition that has ultimately occurred.
Figure 7 - A myriad of small channels as a result of sediment aggradation and salt cedar establishment characterize much of the Forgotten River Reach. This segment lies upstream of Indian Hot Springs, (Photo taken January 2007). Areas like these comprise thousands of acres and are responsible for attenuation of flows (return to table of contents)
Figure 8 - Below Candelaria, the Forgotten River Reach has a narrow, defined channel (foreground) at low flows, but transforms into a myriad of channels through an aggraded floodway at high flows, (Photo taken looking upstream January 2007
Figure 9 - An avulsion out of a salt cedar-armored and aggraded channel into an adjacent Bermuda grass pasture upstream of Indian Hot Springs demonstrates the tentative nature of the channel through the Forgotten River Reach. (Photo taken January 2007)
Channel location changes occurred at 25 places during the flood of 1942 and spill at Elephant Butte Dam. Changes were mainly cutting new channels across 25 bends. In the following years, with much reduced flows and cumulative deposition of sediments in the channel and encroachment of salt cedar, changes began to occur in the channel location. During the years 1967-75, the river at eleven places spread out to flow in a number of small distributary channels, no one of which could be identified as the main channel. In 1978, there were seven additional locations identified where the channel has lost its identity.
As recently as 2003, Fullerton and Batts characterized the Fort Quitman to Candelaria reach as having a sinuosity of 1.5 with a predominately sand bed; a floodplain varying between 0 and 3,000 feet, but typically about 100 feet wide; a channel width varying from 50-100 feet, and a two year discharge flow peak (50% exceedance) of approximately 2,500 cfs (but which varies from 3,800 cfs at Fort Quitman to 1,000 cfs at Presidio). (return to table of contents)
3.3 Sedimentation Observations within the Forgotten River Reach
3.3.1 Limitations
Caveats: The following discussion is based on extremely limited information, primarily consisting of infrared aerial photography viewed within a GIS environment to which a number of “sediment cones” (arroyo confluences) were marked, along with some notes at each cone (based on interpretation of three sets of aerial photography) (UTCSR, 2006(?). It is important to note that this discussion comes about without the benefit of any field work, and is almost entirely based on (potentially erroneous) interpretation of the 2005(?) photography available, coupled with a general understanding of river mechanics and sediment transport, since little quantitative information was available. As such, any recommendations made, or conclusions drawn, should be prudently reality checked prior to proceeding with any project alternative development.
3.3.2 Introduction
The geomorphic responses of river systems can often be understood in terms of energy. The particles of soil (sediment) that make up the boundaries of the river channels and pass through the system respond to the energy of the fluid (water) interacting with them. Larger particles require more energy to mobilize and stay in motion, while smaller sizes may remain suspended in the water column with considerably less energy. In this way, channels form and change, banks erode, bars and floodplains build.
Of particular interest in the current study is the discontinuity of hydraulic conditions typical of confluences. The cessation of the bankline confinement of a tributary channel at the confluence allows the flow field to widen, dispersing energy. A ‘fan’ or ‘delta’ deposit at the confluence is typical of this phenomenon. This effect is often elevated by flow escaping an unvegetated, reasonably-defined channel and encountering obstructions within overbanks (e.g., increased vegetation) that further reduce flow energy. The degree that this type of feature develops is dependant on many individual site factors, such as local geometry, distance of tributary ‘mouth’ from mainstem channel, floodplain vegetation, etc., as well as the amount of energy within the mainstem to redistribute the sediment deposition.
Another useful concept for understanding river systems is response to regime change. This concept is captured succinctly by Lane’s Relationship:
where; QS is sediment discharge
D is sediment size (diameter)
Q is water discharge
S is stream slope
The right side of this proportionality could be considered as the available energy from the amount and slope of the flowing water. The left side could be considered as a response to the right side terms, though responses can occur on both sides of the proportionality.
From this proportionality it is seen that, for example, a decrease in water discharge (Q) (without any slope change, S) would lead to a decrease in sediment discharge (transport through the system, QS) and/or sediment size (D). Thus, sediment typically delivered by tributary arroyos to the mainstem Rio Grande, which historically had higher flows (IBWC, 1978, Teasley, 2005) and consequently more energy to move the sediment through the reach, would be expected to exhibit more deposition, at least for larger sizes, under the current reduced-flow regime. Thus, the mainstem active channel flow area would be expected to decrease, via the width narrowing, the bed aggrading, coarsening, etc. at the macro scale.
These responses do not necessarily occur uniformly throughout the system but, rather, could be associated with where localized imbalances occur, with the expected responses described above typically diminishing in proportion to the distance from the local unbalancing stimuli (e.g., arroyo confluence).
This forms the primary basis for understanding the sediment cones identified within the GIS product, which are almost all in close proximity to tributary confluences, as seen in the individual cone notes. The most plausible explanation for the increased prominence of these depositional zones is primarily as a result of the decrease in the water discharge (and, consequently, energy) of the mainstem Rio Grande in the Forgotten River Reach in historic terms, resulting from water supply infrastructure operations upstream (e.g., Elephant Butte dam), while the inflowing sediment load has remained essentially the same.
To be clear, sediment deposits at tributary confluences are the norm – increasing and diminishing over time in response to the (delivering) arroyo’s and (transporting) river’s hydrology and associated hydraulic energy. The energy regime and, thus, quasi-equilibrium condition, under which the Forgotten River Reach historically formed has been disturbed by the hydrologic impacts of the infrastructure, and the sediment cone behavior is simply part of the river’s response to this perturbation. (return to table of contents)
3.3.3 GIS Assessment
A GIS database prepared by the University of Texas Center for Space Research (UTCSR) was used in this assessment. The database included 56 arroyo confluences (sediment cones) identified visually for three time periods: 2005, 2004 and the mid-1990s. The shape file’s “Notes” for each point included supplemental information, including an ‘activity level’ designation of “Low”, “Medium” or “High” indication of the sediment deposition. The identified sediment cones were viewed within ArcMap against underlying recent (2005?) infrared aerial photography. (See Appendix A)
Overall, the activity designations for the 56 sediment cones were skewed towards the “High” end, with approximately 54% categorized as such (Figure 10). Of the 56 sediment cones, approximately 43% are associated with tributaries flowing in from the US side of the river, with the remaining roughly 57% joining the Rio Grande from the Mexico side. Assessed by origin country, the Mexican tributaries showed a significantly higher proportion designated as “High” activity level (~61%) vs. the US (~39%). Conversely, the Mexican tributaries had a lower percentage designated as “Low” (~10%) than the US tributaries (~17%). Those designated as “Medium” for the US and Mexico were ~44% and ~29%, respectively. (See Figures 11 and 12 below) These differences could potentially be related to topography, surface geology, (recent) hydrology, etc. or could reflect more systematic influences such as differences in grazing practices and watershed management by the two countries.
The sediment cones marked along the study reach appear visually to be randomly distributed for the most part, with the exception of a ‘cluster’ (Feature ID’s 3-14, 54, 55) at about 20% downstream from the top of the reach (see Figure 13). The cluster identified above may be related to a visually obvious topographic feature that dissects it – what appears based on the ‘shadowing’ to be a series (one of which is quite prominent, defined near FID 8) of steep ridges/cliffs that run roughly perpendicular to the general river trend in this area (fault lines?).
Figure 10 - Activity level distribution, overall (return to table of contents)
Figure 11 - Distribution by activity level, US
Figure 12 - Distribution by activity level, Mexico (return to table of contents)
Figure 13 - Spatial distribution of "sediment cones" over study area (return to table of contents)
go to chapters:
1. Overview
2. Study Area Description
3. Geomorphology and Sediment Cones
4. Environmental Resources
5.Problems and Opportunities
6. Recommendations
7. References
8. Appendices
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