Much land along the river valleys is irrigated by means of dams, called by the Igorot “lung-ud’.”  During the season of 1903 there was one dam (designated the main dam in Pl.  LVII —­ see also Pls.  LV and LVI) across the entire river at Bontoc, throwing all the water which did not leak through the stones into a large canal on the Bontoc side of the valley.  Half a mile above this was another dam (called the upper dam in Pl.  LVII) diverting one-half the stream to the same valley, only onto higher ground.  Immediately below the main dam were two low piles of stones (designated weirs) jutting into the shallow stream from the Bontoc side, and each gathering sufficient water for a few sementeras.  Within a quarter of a mile below the main dam were three other loose, open weirs of rocks, two of which began on a shallow island, throwing water to the Samoki side of the river.  In the stream a short distance farther down a shallow row of rocks and gravel turned water into three new sementeras constructed early in the year on a gravel island in the river.

The main dam is about 12 feet high, 2 feet broad at the top, 8 or 10 at the bottom, and is about 300 feet long.  It is built each year during November and December, and requires the labor of fifteen or twenty men for about six weeks.  It is constructed of river-worn bowlders piled together without adhesive.  The top stones are flat on the upper surface, and the dam is a pathway across the river for the people from the time of its completion until its destruction by the freshets of June or July.

The upper dam is a new piece of primitive engineering.  It, with its canal, has been in mind for at least two years; but it was completed only in 1903.  The dam is small, extending only half way across the river, and beginning on an island.  This dam turns water into a canal averaging 3 feet wide and carrying about 5 inches of water.  The canal, called “a’-lak,” is about 3,000 feet long from the dam at A in Pl.  LVII to the place of discharge into the level area at B. For about 530 feet of this distance it was impossible for the primitive engineer to construct a canal in the earth, as the solid rock of the mountain dips vertically into the river.  About fifty sections of large pine trees were brought and hollowed into troughs, called “ta-la’-kan,” which have been secured above the water by means of buttresses, by wooden scaffolding, called “to-kod’,” and by attachment to the overhanging rocks, until there is now a continuous artificial waterway from the dam to the tract of irrigated land.

Considerable engineering sense has been shown and no small amount of labor expended in the construction of this last irrigating scheme.  The pine logs are a foot or more in diameter, and have a waterway dug in them about 10 or 12 inches deep and wide.  These trees were felled and the troughs dug with the wasay, a short-handled tool with an iron blade only an inch or an inch and a half wide, and convertible alike into ax and adz.