Mr. Justice Blackmun
delivered the opinion of the Court.
Mississippi, prompted by the pendency of private title litigation in the Arkansas courts,1 instituted this origi[290]*290nal action against Arkansas in November 1970. The bill of complaint, which accompanied the motion for leave to file, prayed that the boundary line between the two States, in the old bed of the Mississippi River from the upstream end to the downstream end of Tarpley Cut-off, that is, the Spanish Moss Bend-Luna Bar-Carter Point area where Arkansas’ Chicot County and Mississippi’s Washington County adjoin, be fixed and determined.
The river was originally established as the boundary between the States by their respective Acts of Admission. Mississippi’s Act, 3 Stat. 348 (1817), described the line as “up” the river.2 Arkansas’ Act, 5 Stat. 50 (1836), described the line as “up the middle of the main channel of the said river.” See, also, Arkansas’ Constitution, Art. 1 (1874). Over 50 years ago the question whether there was any difference in the meaning' of these two descriptions was resolved and the boundary was determined to be “the middle of the main navigable channel, and not along the line equidistant between the banks.” Arkansas v. Mississippi, 250 U. S. 39, 43 (1919). That decision was in conformity with the rule of the thalweg enunciated in Iowa v. Illinois,, 147 U. S. 1, 7-8, 13 (1893), and followed, in the absence of special circumstances, in many subsequent cases. See, for example, Minnesota v. Wisconsin, 252 U. S. 273, 281-282 (1920); New Jersey v. Delaware, 291 U. S. 361, 379-380 (1934); Arkansas v. Tennessee, 310 U. S. 563, 571 (1940).
Arkansas responded to Mississippi’s motion and moved that leave to file be denied and that the complaint be dismissed. The motion for leave to file, however, was granted. 400 U. S. 1019 (1971). Thereafter, the Hon[291]*291orable Clifford O’Sullivan was appointed Special Master. 402 U. S. 926 (1971). The Master’s report eventually issued and was ordered filed. 411 U. S. 913 (1973).3 Arkansas’ exceptions to the report and Mississippi’s response to those exceptions were forthcoming in due course and the case has been argued to this Court.
Prior to 1935 Spanish Moss Bend was on the thalweg, or primary channel, of the Mississippi River. It has not been the thalweg, however, since the Tarpley Cut-off was established about five miles to the east in 1935 by the United States Army Corps of Engineers. The present controversy focuses on what is known as Luna Bar on the eastern bank of the old river at Spanish Moss Bend. The issue simply is whether Luna Bar came into being by gradual migration of the river westward, or, instead, by some avulsive process, also to the westward. Depending on the resolution of this factual issue, legal consequences ensue in line with established principles conceded by the two States to be the law relating to riparian accretion and avulsion. Nebraska v. Iowa, 143 U. S. 359 (1892); Missouri v. Nebraska, 196 U. S. 23 (1904); Bonelli Cattle Co. v. Arizona, 414 U. S. 313, 325-327 (1973). These principles need no reiteration here. It suffices to say that if Luna Bar was formed by accretion, this litigation is to be resolved in favor of Mississippi, and, contrarily, if Luna Bar resulted from an avulsion, the suit is to be resolved in favor of Arkansas.
Upon our independent review of the record, we find ourselves in complete agreement and accord with the findings of fact made by the Special Master.4 Report [292]*29234. We therefore affirm those findings, overrule Arkansas’ exceptions to the Master’s report, confirm that report, and in general accept the Master’s recommendations for a decree.
We deem it unnecessary to outline at length the evidence adduced, or to reproduce here the detailed analysis of that evidence made by the Special Master. We note only that the dissent would regard the case as close because of three factors: (1) certain testimony as to ancient trees on Luna Bar indicated by the presence of three stumps that could not have lived and died there in the last 100 years, (2) some testimony as to soil on the bar “not compatible with the soil that would result from accretion,” post, at 298, and (3) the bar’s “hard core . . . elevation,” post, at 299-300, that coincides with the elevation “on the adjacent Arkansas bank.” These factors, in our view, would be pertinent except that they reflect only the approach and testimony of Arkansas’ witnesses and overlook pertinent and persuasive testimony to the opposite effect from expert witnesses for Mississippi. The latter are the witnesses that the Special Master credited, as do we, in the evaluation of the conflicting testimony.
Arkansas conceded that Mississippi made out a prima facie case of accretion. Tr. of Oral Arg. 19. In addition, the Master was impressed with the total absence of [293]*293any known historical reference to an avulsion in this area that changed the course of the river by the necessary half mile. And the dissent acknowledges, post, at 295, as to how “Mississippi made its case,” and concedes that the testimony “gives force to the argument that accretion formed Luna Bar,” that there was testimony that in the Mississippi River “avulsion would shorten the course of the river, while here the course was lengthened,” and that Mississippi's experts knew of no instance “where avulsion had worked the way Arkansas claims.”
So far as the ancient tree stumps are concerned, Mississippi presented evidence from forestry experts that the forest on Luna Bar was one predominantly of pioneer species with the expected small accompanying, scattered areas of secondary and climax trees, and with no tree more than 37 years old. This is consistent with the first appearance of growth upon Luna Bar depicted in early Mississippi River Commission charts showing the bar to be barren and without vegetation. Report 10. Mississippi's position as to the three particular stumps was that they had been washed in by floodwaters in preceding years; that one had moss on its roots, a condition incompatible with growth in place; and that, at the point where another allegedly was found in 1972, the elevation of the bar was at least 10 feet above what it had been 90 years earlier. Thus the stump necessarily should have been deep in the undersoil of the bar and not on its surface at the time of its removal. Report 11.
The soil composition is purely a matter of conflicting testimony and we are persuaded by Mississippi's evidence. Deep borings, of course, would be below the riverbed, and would be expected to be consistent throughout the area on both sides of the river. And, as noted above, charts of 1882 and 1894, admitted into [294]
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Mr. Justice Blackmun
delivered the opinion of the Court.
Mississippi, prompted by the pendency of private title litigation in the Arkansas courts,1 instituted this origi[290]*290nal action against Arkansas in November 1970. The bill of complaint, which accompanied the motion for leave to file, prayed that the boundary line between the two States, in the old bed of the Mississippi River from the upstream end to the downstream end of Tarpley Cut-off, that is, the Spanish Moss Bend-Luna Bar-Carter Point area where Arkansas’ Chicot County and Mississippi’s Washington County adjoin, be fixed and determined.
The river was originally established as the boundary between the States by their respective Acts of Admission. Mississippi’s Act, 3 Stat. 348 (1817), described the line as “up” the river.2 Arkansas’ Act, 5 Stat. 50 (1836), described the line as “up the middle of the main channel of the said river.” See, also, Arkansas’ Constitution, Art. 1 (1874). Over 50 years ago the question whether there was any difference in the meaning' of these two descriptions was resolved and the boundary was determined to be “the middle of the main navigable channel, and not along the line equidistant between the banks.” Arkansas v. Mississippi, 250 U. S. 39, 43 (1919). That decision was in conformity with the rule of the thalweg enunciated in Iowa v. Illinois,, 147 U. S. 1, 7-8, 13 (1893), and followed, in the absence of special circumstances, in many subsequent cases. See, for example, Minnesota v. Wisconsin, 252 U. S. 273, 281-282 (1920); New Jersey v. Delaware, 291 U. S. 361, 379-380 (1934); Arkansas v. Tennessee, 310 U. S. 563, 571 (1940).
Arkansas responded to Mississippi’s motion and moved that leave to file be denied and that the complaint be dismissed. The motion for leave to file, however, was granted. 400 U. S. 1019 (1971). Thereafter, the Hon[291]*291orable Clifford O’Sullivan was appointed Special Master. 402 U. S. 926 (1971). The Master’s report eventually issued and was ordered filed. 411 U. S. 913 (1973).3 Arkansas’ exceptions to the report and Mississippi’s response to those exceptions were forthcoming in due course and the case has been argued to this Court.
Prior to 1935 Spanish Moss Bend was on the thalweg, or primary channel, of the Mississippi River. It has not been the thalweg, however, since the Tarpley Cut-off was established about five miles to the east in 1935 by the United States Army Corps of Engineers. The present controversy focuses on what is known as Luna Bar on the eastern bank of the old river at Spanish Moss Bend. The issue simply is whether Luna Bar came into being by gradual migration of the river westward, or, instead, by some avulsive process, also to the westward. Depending on the resolution of this factual issue, legal consequences ensue in line with established principles conceded by the two States to be the law relating to riparian accretion and avulsion. Nebraska v. Iowa, 143 U. S. 359 (1892); Missouri v. Nebraska, 196 U. S. 23 (1904); Bonelli Cattle Co. v. Arizona, 414 U. S. 313, 325-327 (1973). These principles need no reiteration here. It suffices to say that if Luna Bar was formed by accretion, this litigation is to be resolved in favor of Mississippi, and, contrarily, if Luna Bar resulted from an avulsion, the suit is to be resolved in favor of Arkansas.
Upon our independent review of the record, we find ourselves in complete agreement and accord with the findings of fact made by the Special Master.4 Report [292]*29234. We therefore affirm those findings, overrule Arkansas’ exceptions to the Master’s report, confirm that report, and in general accept the Master’s recommendations for a decree.
We deem it unnecessary to outline at length the evidence adduced, or to reproduce here the detailed analysis of that evidence made by the Special Master. We note only that the dissent would regard the case as close because of three factors: (1) certain testimony as to ancient trees on Luna Bar indicated by the presence of three stumps that could not have lived and died there in the last 100 years, (2) some testimony as to soil on the bar “not compatible with the soil that would result from accretion,” post, at 298, and (3) the bar’s “hard core . . . elevation,” post, at 299-300, that coincides with the elevation “on the adjacent Arkansas bank.” These factors, in our view, would be pertinent except that they reflect only the approach and testimony of Arkansas’ witnesses and overlook pertinent and persuasive testimony to the opposite effect from expert witnesses for Mississippi. The latter are the witnesses that the Special Master credited, as do we, in the evaluation of the conflicting testimony.
Arkansas conceded that Mississippi made out a prima facie case of accretion. Tr. of Oral Arg. 19. In addition, the Master was impressed with the total absence of [293]*293any known historical reference to an avulsion in this area that changed the course of the river by the necessary half mile. And the dissent acknowledges, post, at 295, as to how “Mississippi made its case,” and concedes that the testimony “gives force to the argument that accretion formed Luna Bar,” that there was testimony that in the Mississippi River “avulsion would shorten the course of the river, while here the course was lengthened,” and that Mississippi's experts knew of no instance “where avulsion had worked the way Arkansas claims.”
So far as the ancient tree stumps are concerned, Mississippi presented evidence from forestry experts that the forest on Luna Bar was one predominantly of pioneer species with the expected small accompanying, scattered areas of secondary and climax trees, and with no tree more than 37 years old. This is consistent with the first appearance of growth upon Luna Bar depicted in early Mississippi River Commission charts showing the bar to be barren and without vegetation. Report 10. Mississippi's position as to the three particular stumps was that they had been washed in by floodwaters in preceding years; that one had moss on its roots, a condition incompatible with growth in place; and that, at the point where another allegedly was found in 1972, the elevation of the bar was at least 10 feet above what it had been 90 years earlier. Thus the stump necessarily should have been deep in the undersoil of the bar and not on its surface at the time of its removal. Report 11.
The soil composition is purely a matter of conflicting testimony and we are persuaded by Mississippi's evidence. Deep borings, of course, would be below the riverbed, and would be expected to be consistent throughout the area on both sides of the river. And, as noted above, charts of 1882 and 1894, admitted into [294]*294evidence, show Luna Bar as a dry sandbar with no vegetation.
The claim of similar elevations, too, encounters strong and convincing opposing authority. Dr. Charles R. Kolb, a highly qualified expert for Mississippi, testified that his study disclosed that the Arkansas bank, from the first comparative recordings until fairly recent times, was about 12 feet higher than Luna Bar. Report 15, 19. R. 354-357. And there is an absence of levee formations on Luna Bar, as contrasted with the presence of pre-1860 levees on the Arkansas bank.
We agree with the Special Master’s evaluation of the evidence and conclude, as he did, that Arkansas did not sustain its burden of rebutting Mississippi’s conceded prima facie case, a burden the Arkansas court has described as “considerable.” Pannell v. Earls, 252 Ark. 385, 388, 483 S. W. 2d 440, 442 (1972).
Upon our own consideration and our independent review of the entire record, of the report filed by the Special Master, of the exceptions filed thereto, and of the argument thereon, a decree is accordingly entered.
It is so ordered.
[For decree adopted and entered by the Court, see post, p. 302.]