Sedimentology (2011) doi: 10.1111/j.1365-3091.2011.01272.x

Interplay between river discharge and topography of the basin floor in a hyperpycnal lacustrine delta

CORNEL OLARIU*, ,JANOKP.BHATTACHARYAà,MATTHEWI.LEYBOURNE§, STEPHEN K. BOSS– and ROBERT J. STERN** *Department of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, USA (E-mail: [email protected]) National Institute for Marine Geology and Geoecology – GeoEcoMar, 23-25 Dimitrie Onciul Street, Bucharest 024053, Romania àDepartment of Geosciences, University of Houston, 312 Science & Research 1, 4800 Calhoun, Houston, TX 77204-5007, USA §GNS Science, PO Box 30-368, Lower Hutt, New Zealand –Department of Geosciences, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, USA **Geosciences Department, The University of Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USA

Associate Editors – Paul Carling and George Postma

ABSTRACT Basin-floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the Red River delta in Lake Texoma, USA. The Red River discharge is typically a hyperpycnal plume due to elevated total dissolved solids. Because the river plume is a bottom-hugging hyperpycnal flow, lake bathymetry and topography strongly influence deposition and subsequent delta morphology. In addition to elevated total dissolved solid concentrations compared with Lake Texoma water, the density contrast of the Red River outflow is increased by high suspended-sediment concentrations during high-discharge events. Steep lateral slopes in the Lake Texoma basin deflect hyperpycnal river plumes and, subsequently, change the delta progradation direction before the delta reaches the opposite bank of the lake. Analysis of multi-temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake-bottom gradients, corresponding to the pre-impoundment river thalweg (i.e. bypassing shallow parts of the lake). An analytical model for the hyperpycnal-plume trajectory indicates plume deflection during low-discharge or high-discharge events, towards the deepest part of the basin. The magnitude of plume deflection is a function of river discharge and basin-margin gradients. Plume deflection can vary between 10° and 80° from the channel axis towards the old river thalweg. The high deflection appears in the case of maximum basin side gradients of 12Æ8° and in conditions of low river discharge. During low-discharge periods, the Red River delta builds a lobate shape with multiple terminal distributary channels whereas, during high-discharge periods the Red River delta builds an elongate shape with a single large distributary channel. The elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries. Therefore, the lobate shaped delta is expected to be preserved in the rock record. Keywords Discharge, hyperpycnal, lacustrine delta, Lake Texoma, morphology, Red River, topography.

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists 1 2 C. Olariu et al.

INTRODUCTION progradation direction through time are analysed. Thirdly, the control of basin-floor gradients on This paper examines a delta developed by sedi- steering hyperpycnal flows is analysed using an mentation from hyperpycnal-plume discharges analytical model based on the velocity evolution into an artificial lake. Delta progradation direc- of the river plume. tion and morphology are linked to the interplay Examination of the Red River delta in Lake between river discharge and lake topography. Texoma is especially useful because the main Basin bathymetric influence on sedimentation features of the system – basin morphology, river patterns in clastic deposits is recognized widely discharge, sediment load and lake level – are in deep-water turbidites (e.g. Lomas & Joseph, known. Despite the fact that Lake Texoma is an 2004) but studies that demonstrate bathymetric artificial impoundment, the Red River upstream control on deltaic sedimentation are sparse (Wes- of Lake Texoma has a natural regime with min- cott & Ethridge, 1982). Bathymetric control on imum anthropogenic intervention. The main turbidite deposits is obvious, because density contributions of this paper are: (i) quantification currents flow along the basin bottom. For deltas, of the influence of basin-floor topography (pre- bathymetric influence is less important in cases impoundment topography) on delta progradation where river effluent has a hypopycnal character direction; (ii) discussion of the changes in delta- (i.e. buoyed above the basin water; Bates, 1953; plain morphology with river discharge; and (iii) Wright & Coleman, 1974; Nemec, 1995). evaluation of delta progradation rates under Hyperpycnal plumes (i.e. where the river efflu- different discharge regimes. ent sinks below the basin water) for large rivers may be as frequent as seasonal, such as in the case of the Huanghe River (Prior et al., 1986; Mulder & GENERAL SETTING Syvitski, 1995). In spite of being relatively rare events in some modern rivers that feed marine Lake Texoma, a large artificial reservoir that was basins (Mulder & Syvitski, 1995), hyperpycnal built for flood prevention, river flow control and flow events deliver large quantities of sediment to hydroelectric power, is located on the border basins compared with periods of normal hypopyc- between Texas and Oklahoma in the south- nal flow (Warrick & Milliman, 2003). Bay-head, central USA (Fig. 1A). Lake inputs are dominated fjord-deltas and glacio-lacustrine deltas are envi- by the Red and Washita Rivers. Several small ronments where conditions for hyperpycnal flows creeks enter the lake, although these do not have are common because of high sediment loads and major hydrological significance. the low temperature of river waters. Bathymetric Following impoundment in 1944, Red River influence of delta deposits in fjord environments and Washita River discharges inundated the river has been suggested previously (Gustavson, 1975; valley, forming a long and narrow lake (Fig. 1B). Gustavson et al., 1975; Syvitski & Farrow, 1983; The lake has an area of 588 km2, a maximum Hansen, 2004), but there is a lack of research that length of ca 70 km (along the thalweg) and a links delta progradation direction and underlying maximum depth of 34 m near Denison Dam. topography. Delta depocentre migration and major Water volume averages 3Æ29 · 109 m3 (US Army distributary avulsions are other important aspects Corp of Engineers http://www.swt.usace.army. of delta systems that are insufficiently addressed. mil/PROJECTS/civil/civil_projects.cfm?number= The understanding of basin morphological con- 21; Fig. 1B). The Red River contributes two thirds trols on delta progradation might advance under- of the water to the lake with an average discharge standing of delta development (delta depocentre into the lake of 91 m3 sec)1, whereas the Washita migration and channel avulsions). River contributes at an average rate of The Red River delta has resulted from deposi- 48 m3 sec)1. The Red River originates from Tierra tion under the influence of hyperpycnal dis- Blanca Creek, New Mexico, and discharges into charges into Lake Texoma, a large (588 km2) the Mississippi River; it is over 900 km long artificial reservoir bordering Texas and Okla- upstream of Lake Texoma, with a 79 700 km2 homa, USA. Firstly, it is shown that long-term drainage basin (USGS Surface-Water; http:// monitoring data (dissolved and suspended solids waterdata.usgs.gov/nwis/sw). Easily dissolved of river and lake water) are consistent with a Permian evaporites were deposited over a large permanent hyperpycnal flow of the Red River portion of the drainage basin (Fig. 1A) and these into Lake Texoma (Olariu, 2005). Secondly, the crop out especially along the Red River valley. morphology of the delta plain and changes in Interaction of ground water and river water with

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 3

B 101°W 100°W 99°W 98°W 97°W Red River drainage 36°N Washita River drainage Oklahoma Permian evaporites Lake Texoma 35°N

Washita River Area below

34°N

Texas

0 50 100 200 km

C Area of images from Figure 5

Figure 8 Red River Lake Texoma Delta

Denison Dam Red River

~ 30 km to Gainesville A Canada (USGS gauge station) USA Oklahoma Mississippi New Mexico River N 2 km Texas Mexico

Fig. 1. Study area location. (A) Red River and Lake Texoma location within USA. (B) Red River and Washita drainage basins. Area with significant Permian evaporites is taken from geological maps of Texas and Oklahoma. (C) Lake Texoma. Dark grey shows growth of the Red River delta (1945 to 2004). these evaporites results in a relatively high total system have been recorded. Red River hydro- dissolved solid (TDS) load (<500 to 5000 mg l)1) graphs show high discharge during the spring in the Red River. The high TDS has restricted months of May and June and, in some years, high human development along the Red River and autumn discharges in October. Temperatures of results in water with a higher than normal density the lake and river water have similar trends. The (up to 1Æ005 g cm)3) without considering sus- Red River at Gainesville has maximum tempera- pended sediments. This study of the Red River tures between 25 to 35°C during July and August, delta and the plume it forms in Lake Texoma and minimum temperatures of 0 to 10°C during represents a 60 year long investigation in which December and January. The lake has some ther- relevant data (Red River discharge, lake level, mal inertia and the temperature rises slightly later suspended sediments and dissolved solids) of the in the summer, reaching a maximum of 20 to 30°C

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 4 C. Olariu et al. in July/August and a minimum of 5 to 10°C in 1995). Band 1 penetrates clear water deeper than January. band 2, which penetrates deeper than band 3. The different bands of ASTER imagery were compared to evaluate the position of the turbidity front at METHODOLOGY AND DATA USED different depths in the subaqueous delta-front area (Olariu, 2005). The analysis of imagery Vertical aerial photographs and satellite images, (September 2000, June 2001, September 2002 river discharge measurements, pre-impoundment and August 2004) reveals differences in turbidity topographic maps, bathymetric surveys, physical at various water depths and has permitted differ- measurements [discharge, TDS, suspended-sedi- entiation of the position of the relatively turbid ment concentration (SSC)] and water samples in river water and lake water interface thus differen- front of the delta distributary have been used to tiating hyperpycnal flows from homopycnal and study and quantify the Red River delta prograda- hypopycnal flows (Table 1, Fig. 2; see also Olariu, tion and river-plume dynamics since Lake Tex- 2005). Images used have a resolution of 15 m, thus oma was impounded in 1944 (Fig. 2). The data permitting observation of delta progradation, as collected were used to study: (i) the type (hypo- well as morphological changes in the subaerial pycnal or hyperpycnal) of the Red River discharge delta. Morphological changes focused on: (i) delta plume; (ii) the area of the delta plain and the shape; (ii) location, number and size of dis- advance of the channel mouth during delta tributary channels; and (iii) presence of active progradation; (iii) the changes in delta morpho- distributary channels relative to the pre-existing logy; and (iv) the magnitude of plume deflection drainage network. A temporal succession of aerial due to lake bottom topography. photographs and satellite images were used to calculate delta-plain area and ‘linear’ prograda- tion rate (i.e. the rate at which the river mouth Aerial photographs and satellite images advanced into the lake). Morphological changes Bands 1, 2 and 3 of ASTER (Advanced Spaceborne that occurred between image acquisition dates Thermal Emission and Reflection Radiometer) were documented. The area of the subaerial delta satellite images were used to measure the turbid- (delta plain) was adjusted for each image accord- ity of the river plume in front of the Red River ing to lake level on the day that the image was delta (Olariu, 2005). Each band has a different captured. Sediment compaction was expected to wavelength and energy spectrum; thus, each band be small because of the relatively thin deposits penetrates water to different depths (Gordon & (maximum 15 m) and relatively short-time inter- McCluney, 1975; Kowalik et al., 1994; Baban, val (tens of years). The lake level was registered

Time (years) 1945 1950 1960 1970 1980 1990 2000

Water samples for plume suspended sediments

Delta front bathymetry

Lake bathymetry (pre-1944 topo maps) River TDS measurements River SSC measurements ACBMD E F H I J K L Delta images*

Discharge * Letters represent images in Figure 5. Fig. 2. Data used in this study and the time intervals when data were collected (thick black line). The physical parameters were not registered continuously but only for short periods, total dissolved solids (TDS) in 1965, 1977 to 1986 and 1995, and suspended-sediment concentration (SSC) from 1975 to 1987. The smallest black line represents a single day. For the exact dates see the text and Table 1.

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Ó

01TeAtos ora compilation Journal Authors. The 2011 Table 1. Red River delta characteristics measured from successive aerial and satellite images.

Lake River discharge since elevation (m) last image acquisition Delta progradation

Relative River to the discharge Date of conservation during image Peak Delta-plain image pool acquisition Average discharge area Delta acquisition Absolute (188Æ1m) (m3 sec)1) (m3 sec)1) (m3 sec)1) (km2) morphology Area (km2) Linear (m) Image type 21 Nov 1952 183Æ8 )4Æ33Æ28 86Æ1 3 peaks Not Subaqueous delta – – B/W aerial since over 1000 recorded photograph 1 Jan 1945 Ó

01ItrainlAscaino Sedimentologists, of Association International 2011 20 Oct 1955 187Æ5 )0Æ6 112Æ173Æ7 2 over 1000 Not Subaqueous delta – – B/W aerial recorded photograph 27 Feb 1976 186 )2Æ112Æ77 65Æ4 Not – Lobate with multiple – B/W aerial recorded terminal distributary photograph channels 22 Nov 1976 186Æ4 )1Æ715Æ51 48Æ7 No major 8Æ227 Lobate with 8Æ22 ca B/W aerial peaks multiple terminal (since 6000 photograph distributary channels 1944) formation delta on influence bathymetry Basin 21 Sept 1981 186Æ1 )210Æ22 54Æ9 4 over 1000 11Æ26 Lobate with a single 1Æ68 727 B/W aerial large distributary photograph 7 Mar 1982 187Æ3 )0Æ818Æ57 115Æ7 No major 6Æ65 Lobate with two )1Æ84 810 Colour aerial peaks large distributaries photograph 15 Aug 1984 186Æ1 )29Æ8 107Æ7 1 over 2500 10Æ57 Lobate with three large 2Æ77 553 Landsat 321 3 over 1000 distributaries 19 Aug 1991 187Æ6 )0Æ528Æ1 158Æ9 1 over 6500 11Æ11 Elongate with a large 0Æ06 3330 Landsat 321 1 over 3000 distributary, and a

Sedimentology 9 over 1500 secondary distributary active 17 Feb 1995 187Æ1 )120Æ44 134Æ9 2 over 1500 12Æ3 Elongate with a single )0Æ18 1060Æ3 Colour aerial 1 over 3000 distributary photograph 2 Jul 1997 188Æ30Æ298Æ3 185Æ8 1 over 4000 16Æ28 Elongate with a main 0Æ06 682 Landsat 321 1 over 2500 distributary and a secondary one 19 Aug 2000 187Æ2 )0Æ95Æ671Æ5 1 over 3000 15Æ16 Elongate with a single 1Æ56 111Æ3 Landsat 321 distributary 5 6 C. Olariu et al. daily at Denison Dam by the US Army Corps of Engineers (http://www.swt-wc.usace.army.mil/ DENI.lakepage.html). Due to high absorption by

Image type water, NIR (near infrared) and SWIR (short wave infrared) bands were used to distinguish water from land. For morphological observations, dif- 3 Aster 321 6 Aster 321 ferent bands were used to enhance images of the Æ Æ

42 delta/water contact: band 4 (NIR – 0Æ76 to 0Æ86 lm) ) for 1984 and 2001 ASTER satellite images and ) Linear (m)

2 band 5 (SWIR – 1Æ55 to 1Æ75 lm) for 1991 and 2000 Landsat data were used to enhance water/land 33 71 3231 570 Aster 321 contrast. Æ Æ Æ 0 3 4 ) Delta progradation ) Area (km Historical measurements River discharge measurements at the last gauge station (Gainesville; Fig. 1) on the Red River upstream of Lake Texoma were obtained from the United States Geological Survey (USGS) database (USGS Surface-Water; http://waterda- distributary and a secondary one Delta morphology distributary distributary and a secondary one ta.usgs.gov/nwis/sw; USGS Water-Watch; http:// water.usgs.gov/waterwatch/). Discharge measure- ments from 1934 to the present cover the entire period of delta evolution (Fig. 2). The gauge ) 2 03 Elongate with a main 0444 Elongate, a single Elongate with a main Æ Æ Æ station is ca 30 km upstream, therefore a lag time

Delta-plain area (km of approximately one or two days is expected between the Gainesville station and the Red River )

1 delta. The one to two day delay is important for ) considering turbidity observations on satellite sec

3 images, but the time delay is not critical for

Peak discharge (m determining overall delta progradation through

) time or delta morphology. Physical parameters 1 ) recorded at Gainesville (Fig. 1), include TDS and sec 8 2 over 1000 15 Æ 97 1 over 500 14 53 1 over 500 15 SSC. The suspended sediment grain size ranges 3 Æ Æ 4 63 43 Average (m River discharge since last image acquisition from clay (<2 lm) to medium sand (500 lm) with typical d50 values between fine silt (8 lm) and very fine sand (125 lm). Values were correlated ) 1

) with river discharge at the time of the measure-

sec ment and regression models were used to extra- 33 3 Æ 35 84 Æ 2 polate values of TDS and SSC over the entire River discharge during image acquisition (m period of delta evolution. A USGS topographic map of the Lake Texoma area, published before the impoundment of the

1 m) lake (USGS Topographic map – Denison Quadran- Æ 85 15 Æ

) gle, 1901 – 50 foot contour interval), was digitized 0 ) Relative to the conservation pool (188 and used to extract initial (pre-delta) water depths and estimations of Red River valley slopes that

) represent the initial lake bathymetry. Typical basin 3 1 10 Æ Æ Æ slopes were used for analytical modelling to Absolute Lake elevation (m) quantify the magnitude of plume deflection. Continued ( Field data collection The field data represent direct water plume phys- 20 Sept 2002 187 17 Oct 2004 187 Table 1. Date of image acquisition 3 Jun 2001 188 ical measurements that supplement the historical

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 7 data. To establish the type of river plume (hypo- x and y directions, respectively. The velocities pycnal versus hyperpycnal), suspended sediment along the x and y-axes are given by the equations: concentrations in the river plume were measured K x from water samples collected in 0Æ5 l bottles, v v eÀ h ; 1 x ¼ o ðÞ ð Þ filtered using 0Æ62 lm filters and weighed. Tem- g perature, specific conductivity, TDS, and pH at where K is friction coefficient, K and: ¼ C2 different river discharges and at different loca- tions and depths were measured using a Hydrolab vy Cph sina 2 Quanta Multiprobe [Hach Hydromet, Loveland, ¼ ð Þ ffiffiffiffiffiffiffiffiffiffiffiffiffi CO, USA] fitted with a 100 m cable in September Equation 1 is valid for effluent when only the 2003 and June 2004. friction at the bottom is considered where vx is In order to calculate modern delta-front slopes the average plume velocity at some distance x, vo at different locations and to observe morphology is the initial plume velocity at the river mouth, K of the delta front and prodelta, a detailed bathy- is a friction coefficient that is a function of the metric survey using a Knudsen KEL-320 B/P dual Chezy coefficient (C), x is the distance from the frequency (28/200 kHz) echo sounder (Knudsen mouth and h is the average plume thickness. Engineering Limited, Perth, ON, Canada) was Chezy coefficient values were calculated as 1/6 conducted in 2002. The modern delta-front slope C =1Æ49 Rh /n (McCuen, 1998), where Rh is data were used to estimate the shoreline variation the hydraulic radius and n is Manning’s rough- during lake level changes and to correct the ness coefficient. Equation 1 was given by Wright calculations for the delta-plain area. & Coleman (1974) for frictional plumes without considering friction with the ambient water or diffusion processes. Equation 2 represents the Analytical model velocity of a steady uniform flow down an For estimating plume deflection due to basin inclined plane with an angle a (Allen, 1997). topography, a simple physical model (described From Eq. 1 and Eq. 2 deflection (deviation from below) was used to calculate the trajectory of a the channel axis) at a given distance can be moving hyperpycnal plume on an inclined plane. estimated using the following equation: For the plume trajectory computation, only the axis of the plume that flowed on an inclined plane 1 gx y Cph sina xehC2 3 off the river mouth was considered (Fig. 3). To ¼ v0 ð Þ estimate the plume direction at different loca- ffiffiffiffiffiffiffiffiffiffiffiffiffiffi tions, plume velocity evolution was evaluated Initial velocity used in the analytical model was both along, and normal to the channel axis, in the approximated based on channel dimensions and

Inclined plane (Basin side)

vo DirectionA of a hyperpycnal plume on a horizontal planeB

vx v β y v Horizontal plane Axis of deflected Hyperpycnal plume hyperpycnal plume with initial velocity vo α with initial velocity vo on a plane with inclination α on a plane with inclination α

Fig. 3. Sketch with deflection of a hyperpycnal plume that flows over an inclined (lateral) plane. The plume has initial velocity vo in the x direction and, after a time flowing on an inclined slope will have velocity vx and vy after x and y directions, respectively. The plume direction will be deflected with angle b. See text for the equations that control velocities after x and y directions, and calculated plume direction.

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 8 C. Olariu et al. historical river discharge from the USGS data- plume. Close to the river mouth, vertical SSC base. values did not vary significantly, but further from The delta front aggraded, and the shoreline the river mouth, in front of the delta, the bottom prograded preferentially in the direction of the 1 m water layer had a higher SSC (50 to deflected plume. To approximate the overall delta 250 mg l)1) versus the ambient lake water (SSC progradation rates as a function of discharge, the <10 mg l)1). Integration of the SSC and TDS data volume of suspended sediment at a given dis- with the river-plume geometry confirms that the charge in front of the delta was considered. The Red River had a hyperpycnal plume during the distance over which sediments were dispersed measurements. was estimated to lie between 200 m at low Satellite imagery and field measurements repre- discharge and 5 km at high discharge; these sent sporadically collected data and cannot be values were estimated based on the observations used confidently to document the fact that hyper- of Tye & Coleman (1989a) on hyperpycnal flows pycnal flows are permanent characteristics of Red in Grand Lake, Atchafalaya Basin, Louisiana. The River outflow into Lake Texoma. To assess the thickness of the newly formed bed over the delta type of flow regime through time, the density of slope causes the delta shoreline to prograde. For river water relative to the lake water was estimated the computations, the entire sediment volume based on historical SSC and TDS measurements at delivered to the front of the delta was considered, two USGS gauging stations. Although the Gaines- but the sediment dispersal in the newly formed ville gauge station is 30 km upstream of the lake, bed as a function of sediment grain size was not the TDS and SSC values of river water calculated separated. with equations from Fig. 4 are in the same range as the observed measurements in the field during 2003 and 2004 surveys (Olariu, 2005). The TDS RESULTS and SSC of lake water in front of the Red River Delta (at about 3 km) collected during the 2003 and 2004 surveys (1 to 1Æ2gl)1 TDS and about 10 River plume – hyperpycnal flow to 20 mg l)1 SSC) were in the same range as typical The type of flow is controlled by the relative measurements at Dennison Dam (Hubbs et al., )1 density between river water and lake water (i.e. if 1976; Olariu, 2005; 1 to 1Æ5gl TD and 5 to )1 the river water is denser it will sink and the 500 mg l SSC). Physical properties of river water plume will be hyperpycnal). The type of river measured at the Gainesville gauge station were inflow is important in terms of the influence of compared with lake water measured at the Denn- basin topography. When the river plume is hypo- ison Dam gauge station. Red River water has pycnal (buoyant) or homopycnal (neutral), the higher TDS than lake water at Dennison Dam. influence of basin topography on the flow is Lake water density is lower than Red River water minimal but if the river plume is hyperpycnal due to the influx of less saline Washita River water. (negatively buoyant) the basin topography affects Red River water also has higher SSC than lake the river effluent orientation. Olariu (2005) sug- water. Higher TDS in the Red River water is due to gested that the Red River discharge plume was the presence of dissolved salts formed by dissolu- permanently hyperpycnal. Using the principle tion of Permian evaporate beds in the watershed that different electromagnetic wavelengths from (Sun et al., 2011). Plots of discharge versus TDS visible-near-infrared spectrum penetrate to differ- and SSC show that TDS values decrease with ent depths, hyperpycnal river plumes can readily increasing river discharge, whereas SSC values be differentiated on satellite images at different increase (Fig. 4A and B). Because of the opposite river discharges (Olariu, 2005). TDS and SSC variations with discharge, the river During two lake surveys, water samples were water has a permanently higher density than the collected in front of the delta and analysed for lake water and forms a hyperpycnal plume into SSC values. Physical measurements (temperature, Lake Texoma, particularly during high-discharge 3 )1 specific conductivity, TDS and pH) were also events (Fig. 4C). At discharges over 100 m sec made for a better estimation of the river-plume the high SSC creates conditions for hyperpycnal geometry. The 2003 and 2004 surveys indicated a plumes. For low discharge, the high TDS concen- decrease in suspended sediments away from the trations contribute to the formation of a hyperpyc- river mouth but increased concentration above nal plume. Red River water density is permanently the river bottom, corresponding to a hyperpycnal higher than the lake water (higher than about

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 9

)3 0Æ3gcm ) with frequent density differences over A 8 1gcm)3 (Fig. 4C). 7

Red River delta progradation 6 Delta progradation direction, morphology and 5 rates represent the interplay between river dis- TDS = 7564 Q –0·37 R2 = 0·58 charge and bathymetry. The orientation of termi- 4 nal distributary channels and direction of delta n = 116 progradation with respect to the pre-impound- TDS (g/L) 3 ment drainage network were documented in time 2 series images. Red River delta progradation and evolution into Lake Texoma, as discussed below, 1 focuses on three points: (i) lake bathymetry control on delta progradation direction; (ii) delta 100 101 102 103 104 morphology changes with discharge; and (iii) Discharge (m3/s) rates of delta progradation. B Basin-floor bathymetry influence on 25 delta progradation Successive images of the Red River delta (Fig. 5A to O) show a deflection of progradation direction 20 before the delta reached the opposite shore of the lake. The Red River delta bypassed some reaches of the lake in the north-western area (Fig. 5) 15 where the water depth is >2 m. SSC = 19·2 Q0·83 On images from the 1950s, the delta was mainly 2 SSC (g/L) 10 R = 0·66 subaqueous but preferential sediment deposition n = 505 (more turbid water) can be seen towards the western bank of the lake (Fig. 5A and B). On the 5 1976 images, the delta prograded along the old river thalweg along the western bank, but subse- quently cut through and bypassed a meander 100 101 102 103 104 before it continued to prograde north-eastward Discharge (m3/s) again along the old river thalweg (Fig. 5C and D). Leve´e deposits extended northwards filling C accommodation from the old thalweg in an ‘upstream’ direction. On the 1981 and 1982 images, the main terminal distributary channel is oriented eastwards, taking advantage of the 10 slope from an old tributary valley (Fig. 5E and F). )

Fig. 4. Physical measurements, total dissolved solids 3 (TDS) and suspended-sediment concentration (SSC) of the Red River water (at Gainesville gauge station) and (g/cm 1 Lake Texoma water (at Dennison Dam). (A) TDS variation with discharge in Red River water. (B) SSC variation with discharge in Red River water. (C) The difference between the Red River water and Lake Tex- oma water density (TDS and SSC were considered) through time as a function of discharge. To calculate TDS and SSC as a function of discharge, equations from 0·1 1945 1960 1975 1990 2005 Fig. 4A and B were used. It can be observed that the Time (years) summed values are permanently over 0Æ3gl)1.

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 10 C. Olariu et al.

A 21 November 1952 AB 20 October 1955 Turbid water N N Turbid water Subaqueous delta Lake level 4 m below average Subaqueous delta

Lake shore Lake shore 1 km 1 km Old river thalweg and tributaries Old river thalweg and tributaries

C 27 February 1976 AD 22 November 1976 A High angle orientation distributaries N N Initial delta progradation Multiple distributaries Lobate delta Crevasse-splay deposits Upstream delta progradation

Meander cut off

Lake shore Lake shore 1 km 1 km Old river thalweg and tributaries Old river thalweg and tributaries AE 21 September 1981 AF 7 March 1982 Old lobate delta N N

A large main distributary A large main distributary

Lake shore Lake shore 1 km 1 km Old river thalweg and tributaries Old river thalweg and tributaries

G 4 10 Maximum discharge = 6570 m3/s Time of image acquisition /s) 3 103 M 102 B K I D F J O Discharge (m Discharge 1 H 10 C E A Minimum discharge = 2 m3/s L N 19451950 19551960 1965 1970 19751980 19851990 19952000 2005 Time (years) Fig. 5. Delta progradation and morphology changes on successive satellite and aerial photographs. Aerial images on: (A) 21 November 1952; (B) 20 October 1955; (C) 27 February 1976; (D) 22 November 1976; (E) 21 September 1981; (F) 7 March 1982; (G) Red River discharge for the 1945 to 2005 period, the letters represent the images from the detailed panels in Fig. 5. Note that the scale is logarithmic. Aerial images on: (H) 15 August 1984; (I) 19 August 1991; (J) 17 February 1995; (K) 2 July 1997; (L) 19 August 2000; (M) 3 June 2001; (N) 20 September 2002; (O) 17 October 2004. For the lake level at the time of each image see Table 1.

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 11

AH 15 August 1984 AI 19 August 1991 N N

Sandy mouth bar Secondary distributary channel Main distributary channel

Lake shore Lake shore 1 km Old river thalweg and tributaries 1 km Old river thalweg and tributaries JA 17 February 1995 AK 2 July 1997 Channel levée blocking a tributary valley N N

Active secondary distributary

Main distributary channel A single large distributary

Lake shore 1 km Lake shore Old river thalweg and tributaries 1 km Old river thalweg and tributaries AL 19 August 2000 AM 3 June 2001 N N

Reactivated distributaries

Main distributary channel A single large distributary

Lake shore 1 km Lake shore Old river thalweg and tributaries 1 km Old river thalweg and tributaries AN 20 September 2002 AO 17 October 2004

Active distributary ? Active secondary distributary

Larger right levée Main distributary channel Main distributary channel

Lake shore Lake shore 1 km 1 km Old river thalweg and tributaries Old river thalweg and tributaries Fig. 5. (Continued)

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 12 C. Olariu et al.

On the 1984 image, the delta has two terminal an old tributary channel. The gradient in front of distributaries roughly pointing towards the loca- the delta at stage 4.1 was high because it repre- tion of the old river thalweg (Fig. 5H). On Fig. 5J, sents a channel bank (normal to the channel the delta has a single main terminal distributary direction) rather than down the river thalweg. channel that is placed over the old thalweg. The Stage 4.2 progradation was controlled by the main 1991 image (Fig. 5I) follows a large flood in 1987 river thalweg. In stage 5 there were two prograda- that had peak discharges over 6500 m3 sec)1 tion directions, along the river thalweg (stage 5.1) (Fig. 5G). On images from 1991 to the present, and straight towards an old tributary (stage 5.2). the delta has a single terminal distributary chan- During stage 5.2 the delta prograded beside the old nel that seemed locally to take advantage of old river thalweg in an upslope direction, across the tributary valleys (Fig. 5I to O). On Fig. 5M to O, drainage divide, and into an old tributary channel. one of the small leve´e channels that is oriented It is most likely that this was the result of elevated towards the old river thalweg is reactivated. river discharge (Fig. 5G, see the high-discharge It is concluded that the pre-lake drainage period starting after 1985) during which the network influenced the position of terminal dis- plume entered an area of relatively gentle lake tributary channels and the delta progradation slopes (Fig. 6). However, in the latest images direction. It appears that the delta deviates from (Fig. 5M to O) reactivation of the distributary this pattern after 1990 (Fig. 5I to O) following a associated with the stage 5.1 delta is observed. strong flood in 1987 (Fig. 5G) that caused straight- There may be other causes of the delta deflec- ening of the building delta due to inertia. The tion to the right. The Coriolis effect that diverts explanation is that the gradient differences of the moving fluids in the northern hemisphere to the side slopes of the basin in front of the delta right can contribute to the deflection of delta control delta progradation. The pre-impound- lobes, as was described for the hyperpycnal ment topography, digitized from a pre-impound- Huanghe Delta, Bohai Sea (Wright et al., 1990). ment topographic map represents the initial lake However, the Coriolis effect is important at large (basin) bathymetry (Fig. 6). The position of the scales and on slow moving flows (acceleration is old river thalweg has been taken from the pre-lake in the order of 10)4 m sec)2; e.g. Pond & Pickard, topographic map. The delta mainly followed the 1983) and it may be ineffective in Lake Texoma. old river thalweg (Fig. 5), reflecting the fact that The deflection of the river outflow could also be the hyperpycnal plume followed the steepest due to inflow from northern streams or to deflec- gradient, depositing river-derived sediments tion of the main stream from the northern shore of towards the middle of the lake. If the river plume the lake but, in the Red River delta, it appears that was hypopycnal it would float at the top of the the thalweg is the strongest control on flow. lake water and the position (path) of the sediment load plume would not be influenced by the Delta-plain morphology changes bathymetry (Nemec, 1995). with discharge Figure 7 and Table 2 summarize the delta lobe To understand the evolution of delta morphology, positions (progradation stages) at different times a time series of aerial photographs and satellite relative to the old river thalweg. Successive images of the delta were used (Fig. 5; Table 1). images show that the delta prograded mainly Each image acquisition date is indicated on the along the old river thalweg. However, during the Red River discharge plot (Fig. 5G) allowing a evolution period from stage 1 to stage 2, the delta comparison of discharge to delta outline. Because cut off an old meander and did not follow the old the lake level was different for each image, a river thalweg (Fig. 7). In stage 2, the delta pro- typical slope of the delta front was extracted from graded in two directions, in an ‘upstream direc- the bathymetric survey data (Fig. 8) and the tion’ (stage 2.1) and a ‘downstream direction’ shoreline position was corrected on each image, (stage 2.2). The ‘upstream’ progradation (Fig. 5D considering the conservation (optimum level for and E) refers to the progradation of the delta the dam operations) lake level of 188Æ1 m as the mouth in the upstream direction of the old river average. Some shallow features, such as mouth thalweg (note that on Fig. 5D the flow through the bars (Fig. 8C), appear on images acquired during old river thalweg was from north to south, low lake levels. whereas the delta at that point prograded from Analysis of delta morphology, correlated with south to north). The stage 2.2 delta continued in river discharge history and lake level change, stage 3 as the slope was higher in this direction. indicates major changes in the shape of the delta During stage 4.1 progradation was controlled by plain and the number of distributaries over short

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 13

3 758 000 A Old river thalweg 7·1 Slope measurements 4·1 − −4 −2 −3 −5 −6 −9 −6 −7 −7 −6 −6 −6 −7 4·2 −3

−6 −6 3·1 −7 −9 − 6 −3 −8 5·1 1 −6 3·3 −6 3 754 000 −7 5·2 − −6 − 2·2 8 − −3 2·3 −6 −6 −5

−5 −6 −

7

− −6 3·2 −7 6·1 6·2 −6 −4 −8

2·1 −6 −7 −1 Fig. 6. (A) Lake Texoma initial 6·4 −6

−11 bathymetry digitized from the topo- −7 B 6·3 graphic map (USGS Topographic Location Slope Degrees −6 map – Denison Quadrangle) sur- 1 0·2286 12·87 −9 −7 −11 −6 veyed before lake impoundment. Northing (metres) 3 750 000 2·1 0·0127 0·727 −7 −6 2·2 0·00762 0·436 −10 −6 The dotted numbers indicate points −7 2·3 0·1524 8·665 −3 where the basin slopes were esti- 3·1 0·01088 0·623 −6 −4 −6 3·2 0·0762 4·357 −7 mated. Bathymetry is in metres at −1 3·3 0·0254 1·454 −11 1 m contour intervals. (B) Table 4·1 0·00586 0·335 −6 with values of lake bathymetry 4·2 0·01524 0·873 −3 −7 slopes. For location see Fig. 6A. The 5·1 0·0254 1·454 −8 5·2 0·01905 1·091 −11

−6 measurement locations were chosen 6·1 0·01905 1·091 −12 at representative locations to get the 3 746 000 1 km 6·2 0·01088 0·623 −7 range of the slopes in Lake Texoma, 6·3 0·03048 1·745 −6 −6 −5 and have been calculated over a 6·4 0·02177 1·247 −1 distance of relative constant values 686 000 690 000 694 000 (equal bathymetry contour lines). Easting (metres) periods (Fig. 5; Table 1). Because Red River distributary channels (Fig. 5C). At the beginning floods contribute the bulk of the sediment to the of the lake bend, the delta prograded in two delta, the large peak discharges are referred to, as directions, one towards the north, filling the old well as the average river discharge for the period river thalweg, and another towards the north-east, between images (Table 1; Fig. 5G). which represents a cut-off of the old river mean- Since 1952, three main morphologies are der. On the 22 November 1976 image, the delta observed: (i) an initial subaqueous delta (1950 to morphology was still lobate but more numerous 1955); (ii) a lobate delta (1976 to 1982); and (iii) active terminal distributary channels with mouth an elongate delta (1984 to present). Initially, on orientations at a high angle to each other are the 1952 to 1955 images, despite the lake level observed (Fig. 5D). being 4 m lower than the average (in 1952), the However, on the 21 September 1981 image, the subaerial delta can be observed only on the size of one of the distributary channels increased narrow north–south oriented part of the river; as it took advantage of the slope towards an old this indicates that, at this time, the delta was river tributary valley (Fig. 5E). No morphological mainly subaqueous in the wider part of the lake changes were observed on the 7 March 1982 (Fig. 5A and B). The present authors argue that image (Fig. 5F). The area of the delta had appar- the subaerially exposed land that is seen on the ently decreased because the lake level was higher. 1952 photograph (Fig. 5A) was the subaqueous After 1981, peak discharges increased in fre- delta front because on the successive image in quency and magnitude (frequent discharges over 1955 (Fig. 5B) at an average lake level (about 4 m 1000 m3 sec)1 and higher maximum peak dis- higher than at time of the 1952 image acquisition), charges; Fig. 5G). The 60 year long record of river the same area was under water. discharge, from 1945 to 2005, shows variation in discharge (Fig. 5G) that is more obvious when the Lobate morphology. The lobate delta morpho- yearly average discharge is plotted (Fig. 9C). This logy was observed initially in two 1976 images, variability might be caused by long or short-term but also in the successive images of 1981 and climate change atmospheric variability (Milliman 1982. The 27 February 1976 image (Fig. 5C) et al., 2008). Precipitation, and as a consequence shows a lobate delta with multiple terminal river discharge, in the western USA has been

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 14 C. Olariu et al.

N

Bypassed lake area 4·2 3 4·1 ? 1976 1981– 1984 1991–1995 2·1 5·1 5·2 2·2 Bypassed lake area 1960’s

1952– Secondary 1955 active 1 distributary 63 2000–2004

The main 2 km active distributary 2·1 Delta stage (years observed on images) Land Lake shore Pre-lake river thalweg

Fig. 7. Summary variation of Red River delta progradation direction relative to the old river thalweg. Six main stages were differentiated based on: (i) location relative to old river thalweg; and (ii) relative discharge during the period that one particular stage was formed. Dark to light grey colouring represents stages from old (1950) to young (2003). Diagonal lines represent lake areas bypassed by the prograding delta. See also Table 2 for deposition of the delta stages. linked with cycles of atmospheric variability Elongate morphology. By far the largest dis- (Milliman et al., 2008). The Red River discharge charge observed since 1944 was in June 1987, 3 )1 variation might be related to El Nino variations when river discharge exceeded 6500 m sec for that induce decadal variation in precipitation three days. An elongate delta is observed on the patterns and alternate drought and rainy condi- 19 August 1991 image, showing that the Red tions in North America (NOAA-CIRES Climate River delta extended to the northern side of the Diagnostics Center, http://www.cdc.noaa.gov/ lake (Fig. 5I). On 17 February 1995, the delta was ENSO/enso.current.html). Note the good correla- still elongate along the northern shore with a tion between the large Red River discharge and single large distributary channel prograding in warm years (Fig. 9C and D). As a consequence of the direction of the old river thalweg. Because of overall discharge increase, on 15 August 1984, the the main distributary channel progradation, a Red River delta increased the distributary channel leve´e encloses a previous gulf of the lake channels from two to three, along with a prefer- (Fig. 5J). ential lakeward extension of the channels (Fig. The 2 July 1997 image shows the main channel 5H). Because of the shallow water depth, the flow extended along the northern shore of the lake at the channel mouths is friction dominated. The (Fig. 5K). Another inactive terminal distributary friction dominated channel mouth enhances channel oriented north–south appears to be formation of the mouth bars that bifurcate drowned due to the high lake level (Table 1). the distributaries (Olariu & Bhattacharya, 2006; On the 19 August 2000 image, the delta is Edmonds & Slingerland, 2007). elongated with the main distributary parallel to

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 15

Table 2. Description of delta progradation stages be active as it is elongated about 100 m into the relative to the old river thalweg. lake. Delta Two periods of delta evolution have been progradation distinguished: the first period, before 1981, with stage Description relatively low river discharge (multi-annual aver- age value = 69 m3 sec)1); and the second period Stage 1 Delta progrades following the old after 1981, with relatively high discharge (multi- river thalweg annual average value = 126 m3 sec)1; Fig. 5G). Stage 2.1 Delta cuts off an old meander During low-discharge periods, the delta exhibited taking advantage of an old tributary and infills the thalweg a lobate shape but, during high discharge, the in the ‘upstream’ direction delta was elongated with a single main distribu- Stage 2.2 Delta continues to prograde along tary channel reflecting relatively large SSC loads the old river thalweg associated with high discharge. The period Stage 3 Delta has a lobate shape after a during which discharge was relatively low com- period of low discharge and prises the initial period of subaqueous delta possibly wave reworked deposition and lobate delta morphology (stages Stage 4.1 Because of high river discharge 1 to 3; Fig. 7, Table 2). The second period, that of delta progrades towards an old relatively high river discharge, is reflected in the tributary elongated delta morphology (stages 4 to 6; Fig. 7, Stage 4.2 Delta continues to prograde along Table 2). During the last period (1997 to 2004), the old river thalweg the delta maintained an elongate morphology. Stage 5.1 Delta (a new lobe) diverges The Red River delta in Lake Texoma is a fluvial- towards the old tributary dominated delta (Galloway, 1975) because there thalweg are no tides and the wave regime is minimal due Stage 5.2 Delta progrades parallel with the to the reduced fetch. As a fluvial-dominated old river thalweg, on a former delta, the river discharge (water and sediment) terrace due to high river discharge is the main variable that controls delta formation. Stage 6 Delta progrades beside (parallel to) From the comparison of the multi-annual dis- the old river thalweg taking charge variability and the shape (morphology) of advantage of the slopes of lateral the delta (Figs 5 and 7), there is a good correlation old tributary valleys between discharge and delta morphology. Morphology changes (elongate versus lobate) in fluvial-dominated deltas have been obtained by the northern shore under the influence of high using different sediment cohesion values in discharge combined with the steep gradients of numerical models (Edmonds and Slingerland, one of the old tributaries (the delta prograded 2010). It is difficult to evaluate sediment cohesion south-eastward instead of following the river in the case of Red River sediments but, given the thalweg; Fig. 5L). relatively short time of delta evolution since No significant changes appear on the 3 June 1945, no significant changes are expected. The 2001 image; the delta has a similar morphology to sediment cohesion will control stability/erodibil- the previous year. One difference is the extension ity of the channels, but the channel erosion will of the south channel leve´e by tens of metres, also depend on flow velocity that depends on probably due to preferential deposition on that river discharge. side because of deviation of the river outflow towards the right (Fig. 5M). On the 2001 image, Progradation rates the delta appears to be relatively shorter because This study shows high progradation rates for the the lake level was higher than on the 2000 image delta, with an average of ca 250 m year)1 since and part of it was covered with water. On the 20 lake impoundment in 1944. Progradation rates September 2002 image, the delta appearance is mainly depend on river discharge, which deter- unchanged in comparison with 2001 but the mines sediment load (Fig. 4B). The SSC scatter south leve´e of the main terminal distributary relative to the rating curve (Fig. 4B), especially for channel prograded 500 m (Fig. 5N). The last high discharges, can be caused by variation in the image, acquired on 17 October 2004 (Fig. 5M), timing (how often and which season), the source shows that the delta is still elongated but that the (which part of the drainage basin) and the type secondary terminal distributary channel seems to (rain versus snowmelt) of the floods (Morehead

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 16 C. Olariu et al.

3 759 000 A B 3 N N 2 0·5 1 Red River Delta Profile 3 2 Channels ? Red River 3 757 000

1 2 1 1 3 4 2 2 5

Profile 1 3 3 1 2 2 3·5 3 755 000 4 2 4·5 Profile 3 1 Profile 2 4 4 Northing (metres) 6

6 2 Profile 1 2·5 8 9 3 1 Profile 2 2 3 2 7 5 5 5 3 7 9 4 7 Detail in Figure 8B 1

3 753 000 8 4 8

6 10 9 6 400 m 5 4 9 2 2 Contour isobaths are in metres 1 7 Contour isobaths are in metres 3 692 000 694 000 696 000 698 000 Easting (metres)

C W Profile 1 Area below Mouth bar Channel ? Channel ? E –1 –2 –3 0·0025 Multiple –4 0·0014 Depth (m) 0·016 –5 100 m 0·009 –6 Typical delta front slopes Profile 2 Area below –1 W Channel levée E –2 Multiple –3 0·05 0·003 –4

Depth (m) 0·01 –5 100 m 0·014 –6 Typical delta front slopes

Profile 3 Area below –1 W E –2 –3 Typical delta front slopes –4 0·003 Depth (m) 100 m 0·05 0·01 –5 0·014 –6

Fig. 8. Lake Texoma bathymetry in front of the Red River Delta based on the echo sounder data collected on 12 October 2002. (A) Bathymetry in front of the delta. (B) Detailed bathymetry in front of the main river channel. Note that a subaqueous mouth bar was formed. (C) Echo sounder profiles that show channels and a mouth bar. Typical slope values of the delta front were extracted to correct subaerial delta area for the lake level changes. On the figure the vertical scale is about 20 times exaggerated. et al., 2003). However, the long-term, multi- subaerial delta growth for each image was mea- annual sediment supply would be closer to the sured and the areal increase for each time period rating curve (Fig. 4B). Seasonal lake level changes is plotted in Fig. 9B. (about 1 m) are not an important influence on Corrections for lake level were made consid- overall progradation rates. The area of cumulative ering an average slope of 0Æ01 (0Æ57°) in front of

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Ó Diagnostics the Climate where discharge area located. NOAA-CIRES yearly the was River From in distributary Red active Texoma index. (C) main lake time. (ENSO) the of through of depth Oscillation area mouth Initial Nino/Southern delta (E) River El (http://www.cdc.noaa.gov/ENSO/enso.current.html). Red Center Multivariate Subaerial (B) (D) lake. average. the into channel distributary 9. Fig. 01TeAtos ora compilation Journal Authors. The 2011 e ie et rgaainit aeTxm o h 94t 04pro.()Pordto ftemain the of Progradation (A) period. 2004 to 1944 the for Texoma Lake into progradation Delta River Red

3 Initial water depth at delta location (m) Yearly average discharge (m /s) 2 100 200 Subaerial delta area (km ) Main distributary length (km) 10 15 10 0 0 5 0 5 0 5 E A B Standardized departure C –2 –1 2 0 1 3 Elongate delta Lobate delta 1950 D ENSO warmphase:ElNino

Water depth Increases as delta followed the old river talweg Ó 1960 01ItrainlAscaino Sedimentologists, of Association International 2011 discharge (precipitation)pattern El Nino/LaNinaeffect on LOW 1970 Inferred waterdepthfromimagesandpalaeo-bathymetrymap Time (years) ai ahmtyiflec ndlaformation delta on influence bathymetry Basin Steady increase in channel length Significant area decrease area Significant Overall increase in subaerial area 1980 ENSO coolphase:LaNina High progradation rate progradation High HIGH 1990 Measurements onimages Measurements onimages Low progradation rate progradation Low old rivertalweg off prograded delta as depth water Stable Sedimentology Decreasing trend? Decreasing 2000 17 18 C. Olariu et al. the distributary channel and 0Æ02 (1Æ14°) lateral Analytical model experiments on to the active channel along the delta shoreline hyperpycnal-plume deflection (Fig. 8). Subaerial delta area measurements indi- cate that, in at least two cases, area decreased. Red River delta progradation deflection from the Apparent delta erosion can be explained by: (i) main channel direction occurs because the direc- over-simplification, by choice of a unique delta- tion of the hyperpycnal plume is affected by front slope; (ii) sediment compaction and/or laterally sloping planes defined by the old thalweg dispersal (waves can be over 1 m high; Sublette, of the Red River and its major tributaries (Fig. 3). 1955) after periods of high discharge followed by The hyperpycnal plume is driven by gravity and low discharge; or (iii) spatial variation in lake must flow perpendicular to the old valley slope. level; the lake elevations are reported at Denison Therefore, the plume follows a curved trajectory Dam, which is 45 km (lake length) downstream (Fig. 3). To quantify the plume deflection, it is of the delta. If it is assumed that the water in the considered as a frictional river effluent flow where lake is flowing, and that the water has a hypo- the initial velocity decreases with time and dis- thetical slope of 1 cm km)1 (10)5) for a 45 km tance (Wright & Coleman, 1974). The final plume distance, it could produce an error of ±0Æ45 m in trajectory is computed considering velocity varia- lake level estimation in front of the delta. tions along the x (horizontal) and y (vertical) A linear progradation (advance of the river directions (Fig. 3) using Eq. 1 and Eq. 2. The channel mouth) for different intervals is also resulting plume trajectory is described by Eq. 3 that computed (Fig. 9A). depends on the initial plume velocity and the old Channel progradation rate is positive at all valley side–slope (gradients). For the range of times, although the rate is variable and there is an initial velocity, the possible Red River discharge overall decreasing trend, which can be attributed was considered through a typical channel size that to low discharge and/or increasing basin (lake) varies between 0Æ5 m depth and 100 m width water depth. Delta progradation rates (Fig. 9A at low discharge and 2 m depth and 325 m width and B) are controlled by river discharge (Figs 5G at high discharge. For the range of the side slope, and 9C) and accommodation (lake water depth) slopes were estimated from the pre-impoundment (Figs 6A and 9E). Periods of high rates of progra- topographic map (Fig. 6B). Slopes were found to dation can be linked with an increase in overall vary between 0Æ005 (0Æ33°)and0Æ22 (12°). river discharge (Fig. 9C) and an associated in- In the hypothetical case of low discharge crease in sediment flux. The increase in river (5 m3 sec)1) with an initial velocity of discharge appears to reflect climate variation as 0Æ1 m sec)1, and steep lateral slope (ca 12°), the elevated Red River discharge correlates well with plume will be deflected about 80° from the flow El Nino/Southern Oscillation (ENSO) index direction (dark grey on Fig. 10A). In the hypo- (warm phase; Fig. 9D). Despite delta growth into thetical case of high discharge (6500 m3 sec)1) deeper parts of Lake Texoma, the high prograda- with an initial velocity of 10 m sec)1 and low tion rates for 1984 to 1991 (Fig. 9E) can be lateral slope (0Æ33°) the plume will be deflected ca explained by increasing discharge (Fig. 9C). The 8° (Fig. 10A). In the present model, mixing, relatively steady increase in channel length, dilution and frictional processes that will affect despite low discharge after 1997 (Fig. 9C), can the distance that the river plume flows into the be explained by the relatively constant basin lake were not included. depth (constant accommodation; Figs 6A and 9E). The most frequent Red River velocities are ca The accommodation is relatively constant 1 m sec)1 and the lateral slopes are ca 1° (Fig. 10). because the delta is prograding beside the old In this case, the plume will be deflected about 45° river thalweg (Figs 5K to O and 6). at 200 m from the mouth (Fig. 10A). The deflec- Despite high sediment discharge and average tion increases with distance from the mouth as progradation rates of 250 m year)1, it is esti- the velocity along the channel (x) direction mated that it will take more than 200 years for decreases (Fig. 10A). In both cases of low or high the delta to fill the lake, under present condi- discharge, the sediment delivery by the river in tions. This time span for the lake life was front of the delta will follow the steepest gradient calculated using yearly average volumes of sus- and, therefore, the delta will tend to fill the pended sediment divided by the lake volume. Of deepest parts of the basin first. The modern delta course, if the delta bypasses parts of the lake (since 1997) appears to prograde straight; a pos- (Fig. 7) or increases in discharge, the delta will sible explanation is that the modern delta is reach the dam earlier. prograding over an extended gentle delta front

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 19

0

A Hypothetical plume deflection: During low discharge S = 0·33 dg v0 = 0·06 m/s

During average discharge S = 0·33 dg v0 = 10 m/s S = 0·33 dg v = 1 m/s 200 During flood 0 S = 12·87 dg v0 = 0·06 m/s

S = 12·87 dg v0 = 10 m/s

S = 12·87 dg v0 = 1 m/s

400 S = 1 dg v0 = 0·06 m/s

S = 1 dg v0 = 10 m/s

S = 1 dg v0 = 1 m/s 600 Deflection for 1991–2004

800

Distance normal to channel axis (m) Plume deflection considering recorded discharge 1000 0 500 1000 1500 2000 2500 Distance along channel axis (m)

B N

0 200 1991 400 Metres600 800

0

500

1995 1000 1997 Metres 1500

2000 2000 2001 2002 2500 Channel axis orientation in 1991 2004 Observed Red River channel trajectory Modelled Red River channel trajectory Observed position of channel mouth on images (1991, 1995, 1997, 2000, 2001, 2002, 2004) Modelled position of channel mouth (1991, 1995, 1997, 2000, 2001, 2002, 2004)

Fig. 10. Result of plume deflection analytical model using hypothetical data. (A) Theoretical hyperpycnal-plume deflection for a single flood with the given parameters (minimum, average and maximum) of the Red River/Lake Texoma system. The blue line shows the plume deflection calculated for the 1991 to 2004 discharge period. S = lateral )1 basin slope in degrees, vo = initial flow velocity at the river mouth in m sec , different colours represent different initial velocities; different line patterns represent different slope values. (B) Comparison of an analytical model of plume divergence with the observed Red River main distributary channel position for the 1991 to 2004 period. built during the previous relatively high-dis- However, at low discharge the plume will charge period (1985 to 1997) and the river plume advance eastward into the lake less than during rarely flows over old lake gradients. high-discharge periods. As a consequence, the

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 20 C. Olariu et al. plume will flow less over original thalweg topo- the delta prograded was considered and later, as graphy but over its own more gently sloping the delta prograded, a gentler lateral slope of 0Æ01 prodelta deposits. Because of this, deflection (0Æ57°) was used. Comparison between the mod- will be more probable during high-discharge elled and observed channel trajectories (Fig. 10B) periods (Fig. 11A). The approximation of daily shows similar trends; the differences might indi- progradation at a given discharge (Fig. 11B), cate that other unmodelled processes (for exam- based on volume of sediment dispersed by the ple, flow over variable delta front and basin slope, Red River into Lake Texoma per day, indicates a mixing, dilution, Coriolis force and waves) are wide range from a few centimetres/day to tens of involved in delta deflection. The differences that metres/day. Variation of progradation rates and appear (a more curved, less deflected trajectory) plume deflection are a function of initial velocity are caused by over-simplification of the basin (discharge) and slope (Fig. 11A). slopes and use of a single threshold for the plume The progradational model was compared with deflection (average discharge). the main Red River channel trajectory for the 1991 to 2004 interval (Fig. 10B). Plume deflection was considered only for periods with above-average DISCUSSION discharge. This assumption was made because, at low discharge, the river effluent flows only over Conditions for delta deflection delta-front and prodelta deposits without being deflected by the lake bottom topography. An The Red River delta progradation is controlled by initial 0Æ02 (1Æ14°) lateral basin slope over which basin (lake) bathymetry and also has a hyperpycnal

A Slope increases Initial velocity (a) v = 10 m/s (Discharge) Increases (b) v = 1 m/s

(c) v = 0·1 m/s 1° 5° 10°

Distance Distance Distance

B 9 5 3 (a) Q = 6500 m/s --> v = 10 m/s --> SSC = 10 g/L --> Sed-wtsec = 650 kg/s --> Sed-wtday = 1·34 x 10 kg/day --> Sed-volday = 6·7 x 10 m /day --> progday = 33·6 m/day 6 3 3 (b) Q = 100 m/s --> v = 1 m/s --> SSC = 1 g/L --> Sed-wtsec = 100 kg/s --> Sed-wtday = 7·64 x 10 kg/day --> Sed-volday = 3·82 x 10 m /day --> progday = 0·7 m/day 4 3 (c) Q = 5 m/s --> v = 0·1 m/s --> SSC = 0·1 g/L --> Sed-wtsec = 0·5 kg/s --> Sed-wtday = 3·82 x 10 kg/day --> Sed-volday = 1·91 x 10 m /day --> progday = 0·025 m/day

Unconsolidated The volume delivered into the lake Distinct channel dimensions (width Suspended sediment sediment density was considered to be dispersed over and depth) were considered (325 m, concentrations (SSC) was considered an area of 2 km width and of length 100 m, 100 m and 2 m, 1 m, 0·5 m) were calculated based 3 2000 kg/m 5 km, 1·5 km and 200 m for the for the discharge of 6500 m /s, on rating curves from 3 3 discharge of 6500 m3/s, 100 m3/s 100 m /s, and 5 m /s, respectively. Figure 4. and 5 m3/s, respectively. Lake level A constant slope of 0·002 was considered in front of the delta. Delta Subaerial delta progradation Volume/day

Sedimentary bed Slope = 0·002 Bed height Lake width (2 km) Length of sediment deposits (5 km, 1·5 km and 200 m) for different discharges(6500 m3/s, 100 m3/s and 5 m3/s).

Fig. 11. Delta progradation with discharge under topographic influence. (A) Sketch with variation of progradation direction as a function of basin slopes and discharge magnitude. Note that: (i) higher initial velocities will extend the delta further; and (ii) a steeper lateral slope will have a stronger effect (steer) on the river effluent (and delta progradation). For actual calculations see Fig. 10. (B) Computation of progradation rates as a function of discharge for Red River/Lake Texoma conditions. Computations were made for flood discharge (a), average discharge (b) and low discharge (c). Note that progradation varies over four orders of magnitude from centimetres/day at low discharge to tens of metres/day during the large floods [Q = water discharge (m3 sec)1); SSC = suspended-sediment concentration )1 )1 (g l ); Sed-wtday = weight of the daily sediment discharge (kg day ); Sed-volday = volume of the daily sediment 3 )1 )1 discharge (m day ); progday = delta shoreline progradation (metres day )]. Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 21 character. The results of this study can be applied Most marine deltas have hypopycnal river to delta systems that: (i) can produce hyperpycnal plumes (Bondar, 1970; Wright, 1977; Mulder & flows; (ii) form in basins with significant basin- Syvitski, 1995; Nemec, 1995) but, in some cases, floor topography; and (iii) have large discharge hyperpycnal plumes can form (Wright et al., variability. There are no studies to indicate the 1988; Warrick & Milliman, 2003) due to excess influence of topography on hypopycnal-domi- SSC (Wright et al., 1988; Mulder & Syvitski, 1995) nated deltas, but the hyperpycnal character of and the presence of brackish coastal water. In the Red River effluent most probably will accen- hypopycnal flows, which are common in marine tuate the bathymetric influence on the delta. The deltas (Wright, 1977; Mulder & Syvitski, 1995; mechanism of fluvial-delta progradation is to Nemec, 1995), basin bathymetry has a limited build the mouth bars in front of the distributary influence on delta progradation direction, and channel that will split (Bates, 1953; Wright, 1977; basin processes (waves and currents) are more Olariu & Bhattacharya, 2006; Edmonds & Sling- significant in dispersing sediment. erland, 2007) and, eventually, one of the channels However, hyperpycnal deposits are inferred will become the dominant distributary. The from ancient and modern marine systems (Mul- mouth bar location, and which one of the new der et al., 2001, 2003; Mellere et al., 2002; Olariu distributaries becomes dominant, is probably et al., 2005, 2010; Pattison, 2005; Petter & Steel, controlled by the basin gradients (accommoda- 2006; Zavala et al., 2006; Pattison et al., 2007). tion pattern) in front of the delta. Because hypo- The permanent hyperpycnal river flows into Lake pycnal deltas specifically, and in general any Texoma occur because of higher river water delta type, depend on the accommodation in front density relative to lake water. The density con- of the river, it might be that bathymetry (basin- trast needed for river water to form a hyperpycnal floor topography) is one of the main controls on plume has been reported to be as low as the delta progradation direction, but such a 0Æ0003 g cm)3 in Laitaure Lake, Sweden (Axels- hypothesis needs further investigation. son, 1967), a density difference that is always exceeded in the Red River/Lake Texoma system (Fig. 4C), because of the interplay between high Hyperpycnal deltas river salinity and SSC. Studies of deltas are mainly focused on marine deltas, in either modern (Broussard, 1975; Giosan Basin-floor topography influence & Bhattacharya, 2005) or ancient (Broussard, on delta progradation 1975; Coleman & Wright, 1975; Giosan & Bhat- tacharya, 2005) settings. The main reason for this The influence of slope on marine hyperpycnal focus is that lacustrine deltas generally are flows has been observed in two-dimensional smaller than marine deltas, and are environmen- unsteady models for the Eel River, California tally and economically less important. Lacustrine (Imran & Syvitski, 2000). The model results show deltas are also more ephemeral than marine that Eel River hyperpycnal flows tend to flow deltas, as lakes fill with sediments quickly towards the adjacent Eel Submarine Canyon, (thousands to tens of thousands years). There taking advantage of the steepest gradient. How- are exceptions, for example, where modern large ever, influence of basin topography on hyperpyc- lakes represent remnants of marine basins such as nal flows is also expected to occur in bayhead the Caspian Sea, large glacial lakes such as the deltas where the flooded valleys have steep side Great Lakes, or very deep lakes associated with gradients and closed or semi-closed conditions rifting such as Lake Baikal and lakes in the East create brackish to fresh water bays. Fjords are African rift (Bohacs et al., 2003). another environment with conditions conducive One of the key differences between lacustrine to topographically influenced hyperpycnal flows. and marine deltas is that lake water is typically Fjords have steep lateral side gradients and rivers fresh and, because of this, hyperpycnal river that discharge into fjords have periods of high underflows are common. This problem has been SSC during seasons of ice melt (Gustavson, 1975; addressed by studies of sedimentation from river Hansen, 2004). In deltas that form in shallow bays plumes (Akiyama & Stefan, 1984) and numerical with low relief, like the Atchafalaya (Louisiana), models of river deltas (Akiyama & Stefan, 1984; Colorado (Texas), Guadalupe (Texas) and Wax Kostic & Parker, 2003). Attention was also given Lake (Louisiana) deltas (Donaldson et al., 1970; to river-generated turbidity currents into lakes Kanes, 1970; van Heerden & Roberts, 1988; (Ludlam, 1974; Lambert et al., 1976). Roberts, 1998), a preferential progradation

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology 22 C. Olariu et al. direction will not exist and lobate deltas with sec)1 indicates that this condition is fulfilled. The multiple terminal distributary channels are more morphological variations of the Red River delta likely to form (Olariu & Bhattacharya, 2006). The from lobate to elongate during its 60 year history lobate shallow deltas will prograde through suc- (Fig. 5) are related to the large river discharge cessive avulsions filling the entire low accommo- variations (Fig. 5G) which, in turn, are most dation in low topographic basins rather than probably climate related (Fig. 9). As discussed prograding along a preferential path given by the earlier, the river discharge is the main factor that basin bathymetry. These observations suggest that controls growth of the Red River Delta (there are growth of deltas by hypopycnal flow over no tides and minimal waves). However, fluvial- drowned river valleys should be more important delta morphology might be controlled by other during sea-level highstands than lowstands factors, such as sediment cohesion (Edmonds & (in the case that the previous lowstand incised Slingerland, 2010) or the amount of the prodelta valley was not filled). mud, as in the case of the Mississippi River (Fisk Some studies of basin bathymetric influence et al., 1954). In the case of Lake Texoma, the have been conducted on turbidity flows (Kneller lithology of the drainage area was the same et al., 1991; Kneller, 1995; Amy et al., 2004) but during the observed period (60 years) and most these relate the bathymetry to changes in flow probably the sediment type (sand/shale ratio, or conditions and to different types of deposit. cohesion properties) was also similar, in which Turbidity flows occur on tortuous paths in areas case significant changes in sediment cohesion or of high sea floor topography and variable slopes amount of delta-front mud are less likely. The and, because of this, complex stratal geometries elongation of the delta because of ‘sinking’ of the form (Smith, 2004). The difference between distributaries into thick prodelta muds [as envi- turbidity and hyperpycnal flows is that surge- sioned by Fisk et al. (1954) for the Mississippi] is generated turbidity currents might have higher less likely for the Red River Delta that has only a energy with flow velocities of a few metres/sec modest 10 to 20 m thickness overall. (Vo¨lker et al., 2008), extremely large sediment The calculated average Red River delta progra- volumes (Piper & Aksu, 1987) and might surpass dation rate of ca 250 m year)1 is high, but higher steeper slopes, but larger turbidity flows are a progradation rates have been reported for other more rare occurrence (Piper & Normark, 1983). lacustrine deltas (Tye & Coleman, 1989b), includ- Hyperpycnal flows that are not sustained by ing the Grand Lake Delta (Atchafalaya Basin, steep basin-floor gradients stop at relatively short Louisiana; 2 km year)1) and the Lake Fausse distances (in the order of a kilometre to tens of Point Delta (Atchafalaya Basin, Louisiana; kilometres) off river mouths, as in the case of 500 m year)1). The difference is that the Grand modern Yellow River, Bohai Sea (Wright et al., Lake and Lake Fausse Point deltas are prograding 1990) and the Cretaceous Panther Tongue Delta into 2 m deep lakes, whereas the Red River delta (Utah; Olariu et al., 2010). Basin-floor bathy- builds into 6 to 9 m deep water. Because the metry must be significant in front of the river Atchafalaya basin is shallow, the Grand Lake and over the distance that plumes flow in order to Lake Fausse Point deltas (Atchafalaya Basin, have an influence on sedimentation, as in the Louisiana) fill the entire lake rapidly, but it was case of Lake Texoma. observed that deltas also bypassed some lake areas (Tye & Coleman, 1989a). However, the relatively high progradation rates of the Red River Discharge variability and delta progradation delta appear to be because of plume confinement In general, a delta progrades over its own prodelta along the old river thalweg and restriction of and, as a consequence, the slopes over which new sediment dispersal. sediments are deposited are gentle, with only The similarities between the Red River and small gradient variations. Morphological delta lacustrine deltas in the Atchafalaya Basin reflect changes might appear when the delta progrades similar processes and conditions with common into a narrow basin with steep slopes, such as a hyperpycnal flows within a basin that has a fjord, narrow bay or a flooded valley lake. significant elongate deep region. However, differ- A condition for the delta to ‘feel’ the bathymetry ences appear between morphology and prograda- is for the hyperpycnal plume to flow beyond the tion rates of the Atchafalaya (van Heerden & delta front and prodelta. The large discharge Roberts, 1988), Wax Lake (Roberts, 1998), Grand variation of the Red River of between 2 m3 sec)1 Lake, Lake Fausse Point (Tye & Coleman, and 6500 m3 sec)1 with an average of 100 m3 1989a,b), the Laitaure deltas (Axelsson, 1967)

Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology Basin bathymetry influence on delta formation 23 and the Red River delta. A key difference is that mainly follow the old river thalweg (the deepest the Red River has relatively high discharge within part of the basin) as they prograde. The thalweg a narrow basin with a well-defined deep region, represents the steepest slope available for the whereas for the other deltas (for example, hyperpycnal river plume, except where the delta Atchafalaya, Wax Lake), the rivers discharge into cuts off a meander or follows the thalweg of a wider but relatively shallow basins. Because drowned tributary. these other basins are shallow, frictional pro- During low discharge, hyperpycnal deltas have cesses are more important and these deltas will lobate outlines and distributary channels com- build multiple terminal distributary channels monly avulse due to frictional processes. During (Olariu & Bhattacharya, 2006; Edmonds & Sling- high-discharge periods, such distributary chan- erland, 2007) and have a lobate shape. nels tend to straighten because of flow inertia, but In the Red River delta all three conditions might be deflected because of basin topography. conducive to bathymetric influence on delta pro- The analytical model shows that the plume can be gradation are present: (i) hyperpycnal flows; (ii) deflected more than 80o at low discharges over high significant submarine relief variability (high gra- lateral slopes (12°). The model, as applied to the dients); and (iii) high river discharge variability Red River delta/Lake Texoma system, indicated a (yearly and decadal). Under these conditions, the deflection of 30° for the 1991 to 2004 period that is Red River delta does not build directly in front of similar to deflection observed on the aerial images. the river filling the accommodation but diverges towards the deeper parts of the basin (Figs 5 and 7). However, during the last observation period (1998 ACKNOWLEDGEMENTS to 2004) the delta prograded straight because the divergence angle depends on the interplay This is the University of Texas at Dallas contribu- between discharge and side gradients (Figs 10 tion no. 1086. We would like to thank Sedimentol- and 11). The delta thus bypasses some lake areas, ogy Editor Stephen Rice, Associate Editor George not because of the presence of relatively shallow Postma as well as Simon Pattison, Doug Edmonds water that would be easy to fill, but rather because and an anonymous reviewer who have improved of shallow water regions with low side gradients. earlier versions of the manuscript. CO was sup- ported by an IAS graduate grant for data collection during his PhD at the University of Texas at Dallas. CONCLUSIONS CO is grateful to Nate Miller for discussions on the Red River drainage basin; to Boyan Vakarelov, The Red River/Lake Texoma system has signifi- Adam Franklin, Li Sun and Iulia Olariu for assis- cant discharge variations (between 2 m3 sec)1 and tance in different stages of the fieldwork on Lake 6500 m3 sec)1) and experiences permanent con- Texoma; and to John W. and Ruth Smith who ditions that form hyperpycnal flows, during allowed us to use their trailer and boat. which a delta progrades into a flooded valley. Consequently, the direction of Red River delta progradation is strongly deflected by Lake Texoma REFERENCES lateral slopes. Typically, the delta bypasses shal- low areas to fill the deepest parts of the lake. There Akiyama, J. and Stefan, H.G. (1984) Plunging flow into res- is no data on topography influence on hypopyc- ervoir: theory. J. Hydraul. 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