This action was brought by Bonelli Cattle Company, a corporation, to quiet title to certain lands lying within the bed of the Colorado River in Mohave County, Arizona. The Superior Court on an agreed statement of facts entered judgment quieting title against the State and the judgment was affirmed by the Court of Appeals, 11 Ariz.App. 412, 464 P.2d 999. Opinion of the Court of Appeals vacated. Judgment of the Superior Court reversed.
The issue to be determined is whether ownership of Arizona in the bed and banks of the Colorado River acquired by erosion has been lost because dredging has channelized the water of the river, confining it to a part of the bed.
*467Bonelli claims as the owner the East half of Section 3, Township 19 North, Range 22 West of the Gila and Salt River Base and Meridian. It acquired title by deed dated August 12, 1955. Its grantor traces its title to a patent by the United States dated November 5, 1910 issued to Santa Fe Pacific Railroad Company.
A United States Geological Survey map of this area dated 1902-1903 shows that at that time the river’s east bank was approximately one-quarter of a mile west of Section 3. No evidence was produced as to the course of the river thereafter until 1938, but there seems to be general agreement that during this period the river moved steadily eastward. By April 24, 1938 the river’s channel covered all of the east half of Section 3 except a small portion of the southeast corner.
In the year 1938, the flow of the river was brought under control by the construction of the Hoover Dam, and thereafter water was released only in such quantities as was suitable for downstream irrigation. This discharge of water apparently has not been sufficient to cause any significant erosion or reliction in the river. By a map of the United States Geological Survey, Department of Interior, (the Davis Dam Quadrangle) dated 1950, the river is shown as being nearly three-fourths of a mile wide, flowing in part over the west half of Section 3. In 1959 and 1960 the river was dredged by the United States Department of Interior and the water which had theretofore spread over almost the entire width of the river’s bed was confined to the dredged portion, principally within the west half of Section 4 and only partially within the east half of Section 3. That part of the former channel on the east half of Section 3, over which water no longer flows, is the subject matter of this controversy. The plaintiff, Bonelli, is claiming the exposed portion of the bed of the river.
Ordinarily in the United States the bed of a non-navigable river or stream is owned by the riparian owners. Where the stream forms the boundary between owners, each owns on his side to the middle thread. However, the rule is not always applied to navigable rivers. By congressional enactment, the State of Arizona owns title to the beds of all navigable streams within its borders, 67 U.S.Stat. 30, 43 U.S.C. § 1311; Kansas v. Colorado, 206 U.S. 46, 27 S.Ct. 655, 51 L.Ed. 956. Since the Colorado River is not wholly within Arizona’s borders, but is, itself, the border, Arizona’s title extends from the center of the channel eastward to the river’s high water line. Arizona v. California, 283 U.S. 423, 51 S.Ct. 522, 75 L.Ed. 1154; and see Mumford v. Wardwell, 73 U.S. 423, 18 L.Ed. 756; Pollard v. Hagan, 44 U.S. 212, 11 L.Ed. 565; Morgan v. Colorado River Indian Tribe, 103 Ariz. 425, 443 P.2d 421. As an aside, it may be observed that the boundary between Arizona and Nevada is not the channel’s thread. The boundary has been fixed by agreement between the two states.
It is common knowledge that rivers move sometimes so slowly as to be imperceptible during the lifetime of any single individual, and sometimes so rapidly by floods that the river’s banks visibly disintegrate. Where the change is sudden and rapid, the change is said to be by avulsion. United States v. Claridge et al., U. S. District Court, Phoenix, 279 F.Supp. 87; Hirt v. Entus, 37 Wash.2d 418, 224 P.2d 620. Erosion, on the other hand, is the eating away of the soil by the river’s current, and is a gradual, imperceptible process. Where the river moves by erosion, the boundary moves with the stream, but where the river moves by avulsion, the boundary remains in the center of the old channel, Nebraska v. Iowa, 143 U.S. 359, 12 S.Ct. 396, 36 L.Ed. 186; State v. Jacobs, 93 Ariz. 336, 380 P.2d 998.
From the statement of facts and the exhibits introduced in the instant case, we are unable to say whether the eastward movement of the Colorado River between 1903 and 1938 was by avulsion or erosion. It is to be recognized that avulsive changes *468are generally events so momentous that evidence is available to establish them. In the instant case, the failure to produce such evidence is an indication that the changes in the watercourse were imperceptible. The rule of law is that in the absence of clear evidence to the contrary, the movement of the river will be presumed to be by erosion, Arnd v. Harrington, 227 Iowa 43, 287 N.W. 292; Kitteridge v. Ritter, 172 Iowa 55, 151 N.W. 1097.
It follows from what has been said that as the river moved eastward across Section 3, the thread of its channel moved with it and the title of the State of Arizona to the east one-half of the river bed also shifted correspondingly, — the east high water line of the river finally becoming the plaintiff’s west boundary. As the river moved eastward, engulfing most of Section 3 in the channel of the river, the land to which plaintiff held title decreased so that, by 1938, the owners of the east one-half of Section 3 had, by the operation of natural forces, lost most of it to Arizona.
It is Bonelli’s position that when the United States Government dredged the river in 1959, confining the waters thereof to a portion of the river’s bed so as to prevent unreasonable loss of water by absorption and evaporation, the portion of the bed which was not used for the flow of water reverted to the previous owners. But we do not think so.
A stream is a watercourse having banks and channel through which waters flow, at least periodically. Southern Pacific Co. v. Proebstel, 61 Ariz. 412, 150 P.2d 81. As has been said:
“A watercourse does not lose its character as such by reason of the fact that it is improved by deepening or is artificially controlled, nor because it is used as a conduit to carry other waters. Again, the character of a watercourse is not changed by the fact that a pond is created by a dam. Nor does a watercourse lose its character as such because all the water has been diverted therefrom,, no matter for how long a period * * * nor by reason of the fact that the water has all been dammed at a place far up the stream. * * * ” (Emphasis in original.) Smith v. City of Los Angeles, 66 Cal.App.2d 562, 153 P.2d 69.
Obviously, a river does not have to flow continuously across the whole of its bed to the high water mark in order to avoid a claim by abutting owners to a part of the river’s bed. The channel of a river is the bed of the stream over which its waters run, Benjamin v. Manistee River Imp. Co., 42 Mich. 628, 4 N.W. 483, and the bed of a river is the space contained between its banks, Pulley v. Municipality No. 2, 18 La. 278. Arizona does not lose title to the bed of the river to high water mark simply because the river has been dammed and its water channelized to a part of the bed.
The plaintiff argues what it calls the “artificial accretion” theory, urging that an abutting owner is entitled to the accretion caused by artificial conditions over which he has no control. An accretion is a “gradual and imperceptible accumulation of land by natural causes,” Black’s Law Dictionary. In the instant case, the exposed portion of the channel was not brought into existence gradually or imperceptibly. The exposure was man-made and the channel was uncovered rapidly and perceptibly.
The plaintiff also argues what it calls the “re-emergence” theory. This occurs where a river which formerly eroded an abutting owner’s land recedes so that the eroded jand reappears — a process known as reliction. Reliction is an increase of land by a permanent withdrawal or retrogression of a river. We believe, however, that the dredging of the river is an engineering relocation of the waters of the river by artificial means and is riot a true case of withdrawal or retrogression.
We are not unaware that there are jurisdictions which have held that the land to the low water mark of a river or lake *469belongs to the abutting owners, see e. g., State Engineer v. Cowles Brothers, Inc., Nev., 478 P.2d 159. We think, however, the better rule is that the courts have no authority to give away the title to land which clearly belongs to the state. Where, as here, the abutting owner’s title extends only to the high water mark, and the bed and banks belong to the state, we can find no solid legal reason for concluding that the land below the high water mark to the water’s edge belongs to the abutting owners.
Where a river shifts to a new location as a result of unnatural forces, the state does not lose title to the bed of the stream in the old location. People ex rel. Dept. of Public Works v. Shasta Pipe and Supply Co., 264 Cal.App. 520, 70 Cal.Rptr. 618; Padgett v. Central and Southern Florida Flood Control District (Fla.), 178 So.2d 900; State v. Aucoin, 206 La. 786, 20 So.2d 136; Ray v. State (Tex.Civ.App.), 153 S.W.2d 660; Wilemon v. City and County of Dallas Levee Imp. Dist. (Tex.Civ.App.), 264 S.W.2d 543 (cert. den. 348 U.S. 829, 75 S.Ct. 53, 99 L.Ed. 654).
The judgment of the Superior Court is reversed with directions to enter judgment quieting title in the State of Arizona from the Nevada boundary to the high water mark.
HAYS, V. C. J., ROBERT O. ROYL-STON, P. J., Superior Court, Pima Counr ty, WARREN L. McCARTHY, J., Superi- or Court, Maricopa County, concur.NOTE: Justices JESSE A. UDALL and JAMES DUKE CAMERON having announced their disqualification to sit in the determination of this matter, the Honorable ROBERT O. ROYLSTON and Honorable WARREN L. McCARTHY were called to sit in their stead.