Reconnaissance investigations of the discharge and water quality of the Amazon River

United States Department of the Interior

TABLES

RECONNAISSANCE INVESTIGATIONS OF THE DISCHARGE AND WATER QUALITY OF THE AMAZON RIVER

By ROY E. OLTMAN

When the International Association for Scientific Hydrology (lASH) in 1957 began a program for assessment of river-borne disr~olved solids from all sources carried to the ocear"· the investigators found little published inforiJ.,.ation on the Amazon River. The information on the water discharge of the Amazon River, bf'('.ause of the wide range in published values, d~ il not provide a reliable estimate of average ~nnual discharge upon which a computation of ti"-e annual dissolved-solids load of the river conld be made. On the basis of the scanty inforiJ.,.ation available to the investigators, the Amazon River appeared to supply about 10 perc~nt of the total continental water discharge into the world's oceans. However, if the estimated Amazon River discharge was in error by as great a percentage as appeared probable, the calculated annual salt discharge to the oceans could be seriously affected.

This situation led, in May 1961, to a joint proposal by Luna B. Leopold, then chief hydrologist of the U.S. Geological Survey, War·er B. Langbein, staff scientist of the U.S. Geol~gical Survey, and Professor H. O'R. Sternberg, then Director, Centro de Pesquisas de Geografia do Brasil, Universidade do Brasil, for mear11ring the flow, solute load, and sediment conc,-:mtration of the Amazon River. Professor Sternberg gained the backing of Vice Adm. Helio Garnier Sampaio, Directoria de Hidrografia e N rvega- ~o, Ministerio da Marinha, for logistic support. The Marinha do Brasil would supply the necessary gaging vessel and the Geological Survey would provide the gaging manpower and equipment. Arrangements for the first of three reconnaissance expeditions (July 1963, October-November 1963, and August 1964) to the lower Amazon River were completed in the spring 1963. Space does not permit acknowledgement of the assistance of the many other individuals and organizations that helped in carrying out the reconnaissance work.

Four engineers of the U.S. Geological Survey-Frank C. Ames, Luther C. Davis, George R. Staeffier, and the writer-composed the gaging team, which was most ably assisted by several Brazilian naval officers. Professor Sternberg assisted the team during much of the second expedition.

It is the purpose of this paper to present a few of the notable results and conclusions obtained by the joint expedition for the 6bidos location and other pertinent sites on the lower Amazon River. A similar paper was presented at the "Simposio Sobre A Biota Amazonica" Belem, Para, Brazil, June 6-11, 1966. Information in regard to Selfridge, not available in June 1966, has been added to table 1. All quantities were given in metric units at the Belem symposium. Quantities are expressed here in both English and metric units, with the quantities first listed in the system of units as originally published, measured, or calculated, followed by the equivalent in the other system enclosed in parentheses. A final report, in preparation, will provide information on all the work done, including that in the Manaus vicinity.

Scientists conducting a literature search for -information on the discharge of the lower Amazon River may become confused by the many different estimates of discharge published by investigators of South American or world river discharge.

Table 1 contains a list, from 14 sources, of selected published estimates of some aspect of the Amazon River discharge, such as stream geometry, mean velocity, and total discharge made at the general location of 6bidos or the mouth. The earliest estimate in the list is that of Spix and Martius published in 1831. The latest estimate in the list is that of the eminent hydrologist Maurice Parde published in 1955. One may note the wide range of listed values for the average annual discharge at mouth-a range from 2,400,000 cfs (cubic feet per second) or 68,000 ems (cubic meters per second) by Siemens (1896) to 7,200,000 cfs (204,000 ems) (Military Engineer).

Upon inspection of the source documents one finds such perplexing circumstar~es as the following:

The published estimate for discharge at mouth in the "Military Engineer" is credited to Dr. H. P. Guppy. Upon exarrination of Dr. Guppy's tabulation of discharge of large rivers of the world as found in "Nature," one finds that he credited his value for Amazon River discharge (at mouth) to Elisee Reclus. Thus, the real source of the estimate p•1blished in the "Military Engineer" is Reclus. However, during the compilation of estimates l'y Guppy and those in "Military Engineer," tl'~ original Reclus estimate of 100,000 ems (3,500,000 cfs) for the average annual discharge at 6bidos became 2,458,026 cfs (70,000 ems) at mouth of river in Guppy's table and 7,200,000 cfs (204,- 000 ems) at mouth in the tabh in "Military Engineer." Furthermore, upon examination of Reclus' work one finds the widtl' at the 6bidos narrows quoted as 5,000 to 6,000 ft (1,520 to 1,830 meters) and mean depth quoted as about 250 ft (76 meters), which would provide a cross-sectional area of 1,250,00<) to 1,500,000 sq ft (116,000 to 140,000 square meters). Using Reclus' value of velocity of 8,000 yards an hour equivalent to 2.04 meters per sec')nd, one would compute an average discharge at 6bidos of 8,400,000 to 10,000,000 cfs (237,000 to 286,000 ems). Thus, the whole chain of published estimates-Reclus, Guppy, "Militar;" Engineer"- is a compounding of errors.

One may wonder why estimates only, instead of measurements, of the Ama~~n River dis-

charge at obidos are available. The techniques for gaging large rivers were well developed during the 19th century. Revy reported application of completely satisfactory methods in measurements of the Rio Parana at Rosario made in 1871. Although the maximum depth of the Rosario cross section was only 72 feet (22 meters )-shallow in comparison with the 201-foot (61-meter) maximum depth found in the discharge measurement at obidos on July 16, 1963-the same procedures used for holding the measuring vessel in place at Rosario would have worked at obidos, and the current-meter suspension for measurement of velocities at Rosario would have been satisfactory at 6bidos. After establishment of the river gage at obidos in 1928 by the Brazilian Government and the commencement of daily river-stage readings, the attraction to measure the flow and develop a stage-discharge relation for obidos was stronger, for then the annual flow regime could be completely charted. The task of determining the geometry of a selected cross section on the Amazon River at Obidos was relatively simple. Although complete information is not available on the techniques used by the scientists who have reported measured cross-sectional area, it is probable that the method fully described by Revy, as used on the Rio Parana work for obtaining measured depths, was used on the Amazon River. Depth measurements of a selected cross section are made, following Revy's method, by sounding with a weighted line from a ship drifting with the current. In this way, the sounding line remains nearly vertical because there is only minor current drag on it near the bed. As the ship drifts across the desired measuring section, the sounded depth is observed and the location of ship on the cross section (distance from either bank) is determined by standard surveying methods (sextant readings on flags located on the ends of a measured base line established on shore, or theodolite readings taken to the ship from a shorebased instrument). With great care and replication, a very reliable cross section could be so measured. The measurement of stream velocities could have been obtained by anchoring the gaging vessel to permit observations of subsurface velocities with standard current meters or by using subsurface floats. If a few such measurements, referenced to the obidos gage, had been made to cover the range of discharge from low to high water and an approximate stage-discharge relation established for the obidos location, a much better estimate of the average annual discharge at obidos could have been computed.

Although Jarvis' discharge (table 1) for the Amazon River at obidos is the only tabulated one that is known specifically to be bar~d on rainfallrunoff relationships, the map of world runoff, published by L'Vovich (1945) ancl also based on hydrologic calculations, permits an estimate of the discharge at mouth to be measured. Jarvis, using data then available to him, computed the average annual precipitation for the drainage area tributary to obidos as 62 inches (1,570 mm). He based his average annual runoff estimate on a ratio of runoff to precipitation of 34 percent. The estimate one maJ· derive from L'Vovich's map is equivalent to about 110,000 ems (3,900,000 cfs) for either the obidos or at-mouth location. Precipitatior data in the Amazon River basin are presently much more adequate for assessment of basin average annual rainfall then was the case for Jarvis or L'Vovich. Using Thornthwaite's potential evapotranspiration approach and the presently available climatic data, the writer has calculated average annual runoff for area above mouth as 31.5 inches (800 mm), equivalent to average annual discharge of 5,300,000 cfs (150,000 ems).

As discussed in Geological S~:._rvey Circular

naissance Measurements of July 1963", the lack of data for discharge and dissolv~d solids of the Amazon River hampered work on the project of the International Association of Scientific Hydrology (lASH) for calculation of the salt balance of the oceans. The Am~.zon River was known to be the world's largest in terms of discharge, but the degree of uncertainty in the available estimates of the disch~.rge led to the investigations jointly sponsored by the Brazilian Navy, the University of Bra~~n, and the U.S. Geological Survey. The results c'>llected during the first measurements (July 1963) reported in Circular 486 have been suppJ~mented by results obtained in October-November 1963 and August 1964 and will be discussed in detail in a final report (in preparatio"'l.). The results of the measurements of flow at Obidos are discussed in this section; the measurements of water quality and sediment are discussed in a later section. · The major features of the three measurements of discharge at Obidos are presented in table 2.

The methods used in colle~ting the data for the measurements have been discussed in Circular 486. All three discharge measurements were made at the same cros·s section. The August 1964 measurement used subsurface velocities after loss of 300-pound (136-kilogram) sounding weight and current n1eter on an underwater obstruction led the ~raging party to conclude it would be prudent to conserve the z

en

Figure 1.-Relation of concurrent readings on gages, Amazon River at obidos, Manaus, and Taperinha.

remaining equipment. However, the depths for the measurement were taken with a sonic sounder and the results should be nearly as -4

reliable as are those of the first two measurements. The width of section, rangir~ from 7,410 to 7,500 feet (2,260 to 2,290 m€~ers) is

en

different from that measured by LeCointe (1922)-1,890 meters (6,100 feet)-because the cross section selected for the 1963-64 work was about 114 miles (2 kilometers) downstream from LeCointe's section.

Figure 1 shows the good agreement of the concurrent 1963-64 obidos readings with those from Manaus and Taperinha where daily gage readings were available. The stage graphs showing the relationship of concurrent readings for the periods 1928-46 (overlapping records obidos and Manaus) and 1931-46 (obidos and Taperinha) were prepared by plotting selected concurrent readings for the three gages and fitting the curves to the scatter plot. There can be no question that the 6bidos gage datum was recovered with reasonable accuracy, as the concurrent readings 1963-64 lie on the curves developed from data taken in 1928-46 (1931-46 in the 6bidos-Taperinha comparison). For example, the relationship curve Manaus-Obidos shows that a stage of about 7.5 meters (24.6 feet) would have occurred at 6bidos at the crest of the great 1953 flood, which reached 29.7 meters (97.4 feet) at Manaus. Similarly, the Taperinha-Obidos relation curve shows that a stage of about 7.6 meters (24.9 feet) would have occurred at 6bidos during the 1953 flood, which reached 6.65 meters (21.8 feet) at Taperinha. Discussion with obidos inhabitants who remembered the 1953 flood and showed the survey party the level it reached in the vicinity of the gage location verified the approximate obidos stage of 7.5 meters (24.6 feet) for the 1953 event. Thus, significant evidence indicates reliable recovery of the obidos gage datum used in 1928-46.

A rating curve (curve showing relationship of stage to discharge) prepared for obidos on the basis of the three available measurements of discharge and other data is shown in figure

The stage-discharge relation at obidos may be affected at times by variable slope caused by inflow from the Rio Tapaj6s. Additional discharge measurements will be required to define the magnitude of any backwat~r effects.

Guidance in drawing the curve through the discharge measurements and extending it to- the greatest known stage of 7.6 meter~~ (24.9 feet) was obtained from a study of conveyance and slope. The slope computed by use of the Manning formula with a Manning <''>efficient of 0.020 (derivation of the Manning coefficient from a vertical velocity curve is explained later) varied among the three rr~asurements as follows:

It is the author's opinion that th~ square root of the slope varies linearly with stage at and above the stage of the two higher measurements of discharge. Thus, the slope estimated for a stage of 7.6 meters (24.9 feet) at 6bidos is 16.0x10- • The stage-conveyance relation for obidos does not vary much with stage. Conveyance, K, in English units, may be calculated as follows:

During the three trips for collec~ion of reconnaissance data, limited time did not permit an investigation of the overflow situation at Obidos. Maps (see fig. 4) clearly show the area subject to overflow between the mair channel opposite obidos and the terra firma about 32 km (20 miles) south of Obidos. Th€ flood plain, judging by available maps and F.ir photos, is covered with shallow lakes, s·wamps, scrub trees, and grass, and the drainage channels and

~easrement, (ly I96 3

Figure 2.-Stage-discharge relation, Amazon River at Obidos.

abandoned meanders indicate the localized flow directions during floods. A set of levels run from the water surface as far inland on the flood plain as limited time would permit shows the top of the natural levee and flood plain adjacent to the right bank of the main channel opposite 6bidos to be about at elevation 6.9 meters (22.6 feet) (Obidos gage datum). Thus, significant overflow covering the entire flood plain opposite 6bidos would occur any time the 6bidos stage exceeded 6.9 meters (22.6 feet). It is of interest that the former 6bidos gage observer (Mrs. Platt) made a notation in the gage records that overflow would begin at 6bidos when the river level reached 7.5 meters (24.6 feet).

Although overflow directly opposite obidos might not begin until the river stage reached

g

Figure 3.-Measured cross sections from three discharge measurements, Amazon River at 6bidos.

nel slope, and minimum probable roughness (Manning coefficient 0.030). The result based on assumed worst hydraulic conditions showed that the overflow could be ignored without seriously affecting the accuracy of the estimated maximum discharge. The computation based on best conditions showed that a flow equal to about 10 percent of the 1nain channel discharge might bypass the main channel at a stage equal to that of the 1953 fioo~. It is likely that the actual overflow discharge in June 1953 was somewhere between the two r~sults.

Katzer's interpretation of the overflow situation at obidos is not favorable for accuracy of flow measurements. He wrote (1897): "Unfortunately the narrows at obido.c;; is not suitable for this purpose as long as the entire quantity of water is to be determined, because only a part of the Amazon's total water passes at this point. Another part flows into a number of arms in the lowland north of the Serra do Valaio and helps to fill the large lake, Lago Grande do Curuai, which forms a wide water zone with its numerous lagoons and their connecting channels. This water zone stretches like a bowstring across the main arm of the Amazon River, which bends north, and below whose zenith the city of obidos is situated."

Several strips of overlapping aerial photographs taken on flight lines across the flood plain from east to west and north to south from the terra firma to the Amazon River were available to the writer for study. It is unlikely that the overflow situation intimated by Katzer could exist. However, if great accuracy of measurement of the total discharge of a subsequent flood of the size of the 1953 event is desired, the overflow depths and current velocities should be m~asured and the quantity of flow computed. It is the writer's opinion that the quantity of flow bypassing the main channel at the stage equivalent to average annual discharge at obidos is an insignificant percentage of the main channel flow.

Some questions may be raised about the fact that the two higher discharge measurements were made on a falling river stage and hence the measured flows may be less than would occur at equal stage on ·the rising side of the

Figure 5.-Water level observations showing tidal effect at low flow, Amazon River at 6bidos. Auxiliary staff gage was moved at close of readings on November 20 and not set to same datum for rea~ings of November 21.

hydrograph. A computation made by t~~ Wiggin's formula,

where Qe=discharge corrected for c:':tanging

Qm=discharge measured, U=velocity of flood wave (assumed equal

Se=slope of energy gradient, and ~~=rate of change of stage in feet per

showed the correction to be applied to the July 1963 measurement was less than 3 percent (which can be ignored in view of the reconnaissance nature of the work). The August 1964 measurement had a lesser correction computed for it.

Proof of tidal effect at Obidos was obtained by stage readings at short intervals during the November measurement. (The existence of tidal effect was a moot point based on previous investigations.) The graphs of stage readings taken at lf2-hour intervals on November 20 and 21, 1963, are shown in figure 5.

It should be kept in mind that the 1·eadings were taken during one of the lowest flows of the Amazon River at obidos when the upstream reach of tide effect would be a maximum. During the period 192~6 no stage readin.S?; of less than 0.05 meter (1.6 feet) was recorc~d. The mean stage of the November 1963 discharge measurement is -0.5 meter (-1.6 feet), or one-half meter lower than zero datum. The stage,

to 1.3 times mean velocity),

effect of tide on the obidos stage-discharge relation is considered insignificant by the writer.

On the basis of the recorded gage readings for the period 1928-46 and the rating curve in figure 2, mean monthly discharge has been computed as listed:

The computed coverage discharge at 6bidos for the period is 5,500,000 cfs (157,000 ems). The average discharge computed for 6bidos on the basis of the three discharge measurements and obidos gage readings for the period 1928-46 is thus seen to be more than 50 percent greater than Parde's estimate of 90,000 to 100,000 ems (3,200,000 to 3,500,000 cfs).

Some observations on velocity distribution in selected verticals and in the complete cross section at obidos are shown in figure 6 and 7.

The point velocity observations for the vertical distribution of velocities observed on November 21, 1963, .were measured while the corvette was anchored and should be reasonably free of errors caused by movements of the metering vessel during individual observations. Each point velocity observation is the average determined during a period of 40 or more seconds. The effects of natural stream turbulence are evident in the scatter of the observations about the arbitrarily placed distribution graph. It is evident that, because of the large scale of the turbulence, each point velocity observation should have been derived from a meter run of much longer duration-perhaps as long as 4 minutes. From a study of data from 23 United States rivers ranging in depth from 2.4 to 26.7 feet (0.73 to 8.1 meters), Carter and Anderson (1963) determined that an observation period of 4 minutes for the 20-percent depth location will yield a mean point velocity within 2 percent of the probable true average. As predicted by turbulence theory, the effect of turbulence at the ()bidos section is most pronounced in proximity to the channel bed and it decreases as the distance of observation point above the bed increases.

The vertical velocity curve data in figure 6 and the several other vertical velocity distribution curves developed for other lo~ations and dates at 6bidos verify the essential correctness of the Geological Survey's standard procedure for computing the mean velocity in the vertical as the average of point velocity observations taken at 20 and 80 percent of the total depth. The mean in each vertical for the 6bidos discharge measurements of July and November 1963 was computed from the 20- and SO-percent depth observations-each corrected for movement of the measuring corvette during the period of observation, as explained ir. Geological Survey Circular 486. The mean in each vertical for the August 1964 measurema.nt was obtained by applying an appropriate c0effi.cient to the subsurface velocity.

The distribution of mean velocity in vertical across the 6bidos measuring sect!on for the high-flow measurement of July 1963 is shown in figure 7. The distribution of velo~ities in the section is remarkably uniform, as yrould be expected from Geological Survey experience derived from thousands of discharg·~ measurements made on deep, swift rivers ir the United States with similar uniformity of measuring section and similar streambed conditions.

An analysis based on the verti~al velocity distribution graph and the logarithmic velocity distribution law for wide channels (smooth or rough) follows.

v-V V.1{ where v=observed point velocity (all English

V-mean velocity in vertical, /=Darcy-Weisbach friction coefficient, Y=depth of observation (measured from

yo=total depth of measured vertical. This analysis yielded a Darcy-WeislJach friction coefficient of 0.008, equivalent to a Manning roughness coefficient for the depth investigated of 0.019.

This indication of a relatively sn-~oth bed is borne out by the bed profile shown by sonic soundings and the bed material r~mples obtained. A section of fathometer char~ taken November 21, 1963, during a run up the approximate middle of the channel and crossing the general location of the measured cro"s section is shown in figure 8.

Figure 6.-Distribution of velocity in a selected vertical, Amazon River at 6bidos.

Figure 7.-Distribution of mean velocity in cross section, Amazon River at 6bidos.

....LLI

z ....Q.

.,., ....LLJ

c tuations is unknown.) Experience with sand channels in the United States has led to assignment of Manning coefficients in the range 0.018 to 0.035 for such bed geometry. The size distribution of material determined from bed samples is discussed later.

In contrast to the many published estimates of water discharge for the Amazon River, there is little published information on the suspended-sediment and dissolved-solids loads carried by the flow. Katzer (1897) and Sioli (1957) have published a few analyses of suspended sediment, total dissolved solids, and some other information on the chemical and pltysical nature of the Amazon River water in tl'~ general vicinity of obidos. Pinto computed ~- mean daily discharge of suspended load at mcuth of Amazon to be 3 million metric tons (3.3 million tons).

Undoubtedly, the lack of means for coHection of suspended-sediment samples from yarious depths at the relatively high velocities d~~cour aged investigators from attempting to assess the mean annual suspended-sediment discharge at obidos. Similar difficulties would have discouraged attempts to collect samples of the material in place on the streambed.

The quality-of-water aspects of the investigation were conducted by Mr. F. C. Ames, U.S. Geological Survey, Denver, Colo. Mr. Ames (April 1966) furnished table 3, which shows the results of the three series of samplings made at obidos. The tabulated suspended-sediment concentrations are the calculated average concentration for the cross section. Suspended-sediment samples were taken at many points in each vertical sampled, so that the distribution suspended rna terial in the vertical could be charted. Table 3 also includes information on the range of suspended-sediment concentrations measured in the cross section. As expected, the concentration of suspended sediment is high in the vicinity of the bed.

Ames reports (oral commun., May 1966) this information on the bed material at obidos:

"The median diameter of bed material averaged about 0.20 mm. The median diameters indicated by individual samples ranged from 0.15 to 0.25 mm. Only one to two percent of the bed material (by weight) was finer than 0.062 mm and only one or two percent was coarser than 0.4 mm."

Katzer reported total dissolved solids of 56 mg/1 in a sample taken at obidos June 30, 1896. Because the turbulent mixing and lack of major tributary inflow in the vicinity of obidos (the discharge from the Rio Trombatas would have small effect) should guarantee uniformity of dissolved-solids concentration in a cross section of the stream, the minor differences in the four analyses can be attributed to seasonal variations. One would expect the total dissolved solids found by Katzer on June 30, 1896, to be more dilute than he reported unless the flow for that season 'vas very low. The concentration of dissolved sor~s on August 9, 1964 (discharge 5,810,000 cfs = 165,000 ems), if there were a relatively fixed inverse curvilinear relation between tC't,al dissolved solids and water discharge, woul1 be expected to be slightly higher than the concentration found on July 16, 1963 (discharge 7,640,000 cfs = 216,000 ems). The contrary findings, 21 mg/l and 28 mg/1, respectively, show the importance of seasonal variations in the proportions of total Obidos flow contributed by "white water" and "black water" tribubries. It is apparent that a minimum of one water sample per month collected over several years would be necessary to describe accurFtely the dissolved-solids load variation at obidos.

The dissolved-oxygen content is close to saturation level at the observed stream temperature. Bathythermograph results showed no detectable variation of temperature in any of the verticals where observations were taken from surface to bed and return. The pH was found to be as expected from the analyses reported by Sioli (1957) for the vicinity of Santarem.

Only two large tributaries, the Rio Tapaj6s and the Xingu River, enter the Amazon River downstream from 6bidos. In August 1964, the gaging party measured the dry season discharge of the Rio Tapaj6s at Sao Luis, location of the first rapids, about 180 mi (300 km) upstream from Santarem. The discharge was found to be 99,000 cfs (2,840 ems). A stage-discharge relation for the Rio Tapaj6s gage at Fordlandia was drawn on the basis of the one discharge measurement, measured geometry of the high-flow cross section at Sao Luis, and a consideration of the apparent variation of water-surface slope with stage for the Rio Tapaj6s location at Sao Luis. On the basis of available daily gage readings from the Fordlandia gage and the constructed stage-discharge relation, a mean annual discharge for the Rio Tapaj6s has been calculated as 250,000 cfs (7,100 ems). No measurement was made on the Xingu River.

A low-water-season measurement made by the joint survey group on the Tocantins River (not considered an Amazon River tributary) at Maraba in October 1963 showed the discharge to be 52,800 cfs (1,500 ems). Using the cross-sectional area at Maraba that would be occupied

velocity at bankfull stage, the writer hM calculated a bankfull discharge of 115,000 cfs (33,000 ems). No stage records are available at Maraba. The mean discharge for the Tocanti:M River was estimat.ed, on the basis of the low-water measurement and the estimated bankfull discharge, as 400,000 cfs (11,000 ems).

The Tocantins River is not tn"butary to the Amazon River (its basin has a common drainage boundary with the Amazon River) but its estimated mean discharge and that of the Rio Tapaj6s permit a "bracketing" of an estimated mean annual discharge for the Xingu River. The sum of mean annual discharges for Tapajos and Xingu Rivers, and minor tributaries between 6bidos and the mouth is estimated to be 640,000 cfs (18,000 ems). On this basis, the partly estimated average annual discharge of the Amazon River at mouth is 175,000 ems (6,100,000 ds).

Estimates of the runoff from the intervening area between obidos and the mouth ba~~d on the Thornthwaite potential evapotranspiration approach result in a discharge of abo•xt double that based on hydrometric data. The estimates based on hydrometric data are considered to be more reliable than those based on rainfall-runoff computations. The writer conclude;-- that the most reliable value of average annual discharge for the Amazon River at its mouth is 6,100,000 cfs (175,000 cms)-about 10 times th~ average discharge of the Mississippi River at the mouth.

The results of the joint investigatfons show the previously published estimates of mean annual flow past obidos to be much too low. The average discharge computed on the hu~is of a stage-discharge relation developed from three complete discharge measurements and daily stage readings for the period 1928-4fi is 5,500,- 000 cfs (157,000 ems). The great flocd of 1953 which probably reached a stage of 7.6 meters (24.9 feet) at obidos is calculated to have discharged at 12,500,000 cfs (350,('·10 ems) through the main channel with an indeterminate quantity of overflow on the flooi plain.

The observations of dissolved-8(] lids~ suspended-8ediment, and other water-q,J:ality parameters provide ntuch more information on these aspects for the obidos location than had been determined previously, but therr~ is insufficient information to permit an accur<'1te assessment of either the mean annual suspended load or the salt-load discharge. The bed material samples and fathometer charts provide much insight into the nature of the streaml'~d at obidos.

The objective of the joint investigation to provide reconnaissance information on the flow and water quality of the Amazon P.iver was achieved. If more refitted detertninati ons of the average annual flow and water quality characteristics are needed, it will be necess~,ry to conduct intensive inve8tigationg nt obido~ and elsewhere in the basin. The maintenance of a river-stage gage at obidos-above tidal effects during all but extremely low flow-would provide valuable information at small expense.

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