Riparian and water boundaries

Downstream of the district's new pump intake, four parcels front the Sun River. On the 1958 survey the bank wandered in and out around a gravel bar; today the river has moved south by 40 to 100 m and the old channel is a line of willows in a hay field. The district needs to know where the parcel boundaries are now, because the intake pipe crosses two of them and the pump station sits on a third. The answer depends on rules about water boundaries that are among the oldest in property law and among the most often litigated. This lesson explains them, with the Montana rules and the common-law doctrines behind them.

Natural boundaries

A natural boundary is one formed by a natural feature, most often the edge of a body of water. Unlike a fixed boundary, it is not defined by monuments but by the feature itself, wherever the feature is at the time. When land is granted or surveyed to a river, the parties intend the parcel to run to the water, and the law generally lets the boundary follow the water as it changes gradually. A surveyor dealing with a natural boundary therefore has two tasks: to identify which feature is the legal boundary, by the governing law and the instrument that created the parcel, and to locate that feature as it is on the day of the survey.

Which feature is the boundary is not obvious. A river has a channel, a bank, a high-water line, a low-water line, a flood line and a vegetation line, and each moves differently. The legal boundary is usually one of these as defined by statute or case law, and it is rarely the water's edge on the day of the survey, which depends on recent rainfall, irrigation releases and dam operations. On a river regulated for irrigation, as the St. Mary and Sun rivers both are, the waterline on survey day says almost nothing about where the boundary lies.

In Montana, the state owns the beds of navigable rivers and lakes, a right it took at statehood in 1889 under the federal equal footing doctrine. Montana's statute on owners of land bounded by water says that, unless the grant shows a different intent, an owner whose land borders a navigable lake or stream takes to the edge of the water at the low-water mark, and an owner whose land borders any other water takes to the middle of the lake or stream. Navigability for this purpose is a federal question, decided reach by reach as of statehood. In PPL Montana v. Montana in 2012, the US Supreme Court held that the courts must examine navigability segment by segment, and that the Great Falls reach of the Missouri River, with its falls and long portage, was not navigable at statehood, so the state did not take title to that riverbed. Separately, Montana's 1985 Stream Access Law, following the state Supreme Court's 1984 decision in Montana Coalition for Stream Access v. Curran, lets the public use rivers and streams capable of recreational use up to the ordinary high-water mark, whoever owns the bed.

Accretion, erosion, reliction and avulsion

Four processes change water boundaries. Accretion is the gradual and imperceptible deposit of soil along a bank, which builds new land, called alluvion, against the riparian parcel. Erosion is the gradual and imperceptible wearing away of a bank. Reliction is the gradual retreat of water, exposing land that was once covered, as when a lake level falls over decades. Avulsion is a sudden and perceptible change, such as a river cutting a new channel across a meander in a single flood. The common-law rule, followed in both Canada and the United States, is that gradual changes move the boundary with the water, while sudden ones do not: after an avulsion, the boundary stays where the old channel was.

The reasons are practical. If a boundary could not move with gradual change, a riparian owner would slowly lose access to the water as a strip of new land formed between the parcel and the river, owned by someone else; and an owner whose land eroded would keep title to land under water. Letting the boundary follow gradual change keeps each owner on the water. An avulsion, by contrast, can be seen and dated, so the old boundary can still be located, and it would be unfair for a flood to transfer a whole field from one owner to another overnight. The difficulty in practice is telling which process happened. A river that moved 100 m in 60 years may have done it gradually, or in two or three floods, and the evidence comes from old surveys, aerial photographs, flood records and testimony.

For navigable waters in Montana, accretion and reliction generally add land to the riparian owner and erosion takes it away, while the state's bed moves with the river. Where the river is non-navigable, each owner already runs to the middle, and the thread of the stream moves with gradual change. Islands raise further questions: an island formed in a navigable river may belong to the state, while one in a non-navigable river belongs to the owner on whose side of the thread it formed. These rules are worth checking against current Montana statutes and case law before any decision, because they are frequently litigated.

Dividing accreted land among owners

When a bank moves, the new land must be divided among the riparian owners. The simplest approach extends each side line straight across the new land to the new bank, but that can give some owners a long frontage and leave others with none, especially where the bank has changed shape. Courts have therefore often divided the new bank in proportion to each owner's frontage on the old bank, then drawn new side lines from the old division points to the new ones. The method a court will accept depends on the jurisdiction and the shape of the change, and some courts prefer methods that run lines perpendicular to the general course of the river. Theo computes the proportional division for the four parcels below the intake to show the district what is at stake.

import math
# Accretion along the river: divide the new bank among riparian owners in proportion to old frontage
old_bank = [(0, 100.0), (90, 128.0), (180, 104.0), (280, 150.0), (390, 101.0), (470, 112.0), (560, 108.0), (800, 103.0)]  # 1958 plan
new_bank = [(0, 62.0), (200, 55.0), (420, 50.0), (600, 56.0), (800, 66.0)]                                     # 2026 survey
owners = [("Haugen", 0, 180), ("Kovac", 180, 390), ("district", 390, 560), ("Sato", 560, 800)]

def n_at(line, e):
    for (xa, ya), (xb, yb) in zip(line, line[1:]):
        if xa <= e <= xb: return ya + (yb - ya) * (e - xa) / (xb - xa)

def length(line, e1, e2, step=0.5):
    pts = [(e1 + i * step, n_at(line, e1 + i * step)) for i in range(int(round((e2 - e1) / step)) + 1)]
    return sum(math.dist(a, b) for a, b in zip(pts, pts[1:]))

def easting_at_length(line, target, step=0.5):
    run, e = 0.0, 0.0
    while e < 800 and run < target:
        run += math.dist((e, n_at(line, e)), (e + step, n_at(line, e + step))); e += step
    return e

old_total = length(old_bank, 0, 800); new_total = length(new_bank, 0, 800)
print(f"old bank {old_total:.1f} m, new bank {new_total:.1f} m")
print("owner      old frontage   share of new bank   new line ends at E (proportional | straight south)")
run = 0.0
for name, e1, e2 in owners:
    f = length(old_bank, e1, e2)
    share = f / old_total * new_total; run += share
    print(f"{name:9s}  {f:8.1f} m      {share:8.1f} m          {easting_at_length(new_bank, run):6.1f} | {e2:6.1f}")

old bank 828.8 m, new bank 800.5 m
owner      old frontage   share of new bank   new line ends at E (proportional | straight south)
Haugen        187.4 m         181.0 m           181.0 |  180.0
Kovac         230.5 m         222.6 m           403.5 |  390.0
district      170.8 m         165.0 m           568.5 |  560.0
Sato          240.1 m         231.9 m           800.0 |  800.0

The old bank wandered around the gravel bar, so it was about 829 m long; the new bank is straighter and about 801 m long. Shared in proportion, the Kovac parcel, whose old frontage included the largest bend, ends 403.5 m along the new bank instead of at 390 m where its side line would meet the new bank if run straight south. That 13.5 m difference decides whether the district's pump station sits on Kovac land or on the district's own parcel. Ruth's advice to the district is that the computation shows a real question of law, not an answer: before the station is built, the district should settle the boundary by agreement with the Kovacs or seek a legal opinion on which method applies.

Reservoirs and artificial waters

The district's own storage reservoir raises a different question. When a dam floods a valley, the new shoreline is artificial, and the rules for natural water boundaries do not simply carry over to it. The land under the reservoir was usually acquired by the dam's owner, outright or by a flowage easement giving the right to flood it up to a stated elevation, and the boundaries of that land were surveyed before the water rose, by courses and distances or by an elevation contour. A parcel described as running to the reservoir therefore usually runs to a surveyed line or contour, not to wherever the water happens to be. The district's 1950s acquisition plans for the reservoir show a taking line at a fixed elevation with an added margin for wave action, and Ines will retrace that line from its monuments and its elevation, using the vertical datum of the original survey, rather than from the shoreline. Canals, ditches and drains built by people are likewise artificial features: a parcel bounded by a canal is bounded by the right-of-way line the canal sits in, not by the water.

Locating the bank on the ground

Ines's crew walks the river with the district's environmental scientist. They look for the line where the water has left its mark over many years: where the soil changes from sorted sand and gravel to developed topsoil, where vegetation changes from willow and sedge to upland grass, where there is a scarp or a line of drift. They photograph and survey that line, compare it with the 1958 plan and with aerial photographs from the 1950s onward, and note any signs of sudden change, such as an abandoned channel with a sharp edge. The legal bank they record is a professional judgment supported by that evidence, and the plan says so.

Two practical cautions apply on regulated rivers. First, dams and irrigation diversions change flows, so the marks the water leaves today may differ from those it left before regulation; the legal bank is still where the water has shaped it, but the evidence takes more care to read. Second, a feature built by people, such as riprap or a dyke, does not usually create a natural boundary; the boundary may lie behind or under it. The district's own intake works will need their own plan showing the bank as it stood before construction.