Irrigation water and foundations in the Grand Valley

The valley was farmed the moment water could be brought to it, and it has been irrigated ever since. The Government Highline Canal was completed in 1917, and four separate entities distribute water across the valley today: the Grand Valley Water Users Association, which operates the federal system serving roughly 30,000 acres north of the Colorado River; the private Grand Valley Irrigation Company, covering the remainder of the north side; the Orchard Mesa Irrigation District on the terrace south of the river; and the private Redlands Water and Power Company, which lifts water out of the Gunnison to serve the Redlands.

A water table that people built

US Geological Survey work on the Grand Valley puts it plainly. Irrigation recharge occurs through leakage of unlined canals and irrigation ditches and through flood-irrigation practices. And, more strikingly, the survey found that although the shallow aquifer systems here might not have contained appreciable quantities of water in their natural pre-irrigation state, irrigation now provides substantial quantities of water sustaining a water-table system in the shallow unconsolidated materials.

Read that again in plain terms: the shallow groundwater under a desert valley is largely an artefact of a century of agriculture. It is not a background condition that houses were designed around; it is a condition that arrived with the canals.

The soil survey data corroborates it independently. Wet-season water table depths recorded under Grand Junction include roughly 14 inches in the poorly drained Cojam series, around 36 inches in Bebeevar, and around 39 inches in Green River. Several series carry aquic or oxyaquic wetness classifications, which is a designation for soils that are periodically saturated, in a place that receives nine inches of rain.

The five water sources that reach a Grand Valley foundation

1. Roof water. The largest concentrated volume by far. A modest roof sheds hundreds of gallons in a single monsoon cell, and a downspout discharging at the wall delivers all of it to one spot in the active zone.
2. Lawn and landscape irrigation. The most persistent source. A sprinkler zone running against a stem wall three times a week through the summer puts more water into that soil than the sky does in a year.
3. Canal and lateral seepage. Unlined ditches leak by design and by age. Where a lateral runs close to a property, the soil between it and the house carries elevated moisture through the delivery season and loses it in winter.
4. Flood and furrow irrigation on neighbouring ground. Common on orchard and pasture parcels in Palisade, Fruita, Loma and the rural edges. Water applied to the surface of an adjacent lot moves laterally through the profile.
5. Grade and surface runoff. Ground falling back toward the house, a patio pitched the wrong way, a window well with no drainage.

Why the seasonal shut-off matters as much as the water

Expansive soil damages foundations through change, not through moisture level. The Grand Valley delivers a very sharp change twice a year. Irrigation water is turned on in spring and the soil profile wets up over weeks. It is turned off in autumn and the profile dries down over months. Layered on top of that is the monsoon: the National Weather Service office here says the southwest monsoon generally begins around the second week of July and is over by the end of August in most years, though it can persist into October. So a Grand Valley foundation typically experiences a slow wetting, a mid-summer drying, a burst of intense storm water, and then a long drawdown.

That is a demanding cycle for a clay with smectite in it, and it repeats annually.

Freeze on top of it

NOAA 1991 to 2020 normals for the Grand Junction station record about 137 days a year with a minimum temperature at or below freezing. Freeze-thaw does not move a properly founded house, but it does act on saturated near-surface soil, on flatwork joints that have collected water, and on concrete that is already suffering sulfate attack. A driveway that has been lifted and then left with open joints through a Grand Junction winter will not stay lifted for long.

What to do about it

• Move roof water at least six to ten feet from the wall. Extensions or buried discharge lines. This is the cheapest and highest-return action available to any homeowner here.
• Pull the irrigation back from the foundation. Reposition heads throwing at the stem wall, move drip lines out of the first few feet, and check for leaking valves and laterals near the house.
• Re-establish fall away from the house. The common residential rule of thumb is roughly six inches of fall in the first ten feet. What matters more than the exact figure is that water leaves rather than pools.
• Keep watering steady rather than stopping it. Where a house is already showing seasonal movement, abruptly drying the soil out is as damaging as soaking it. Even the schedule out.
• Know where the laterals are. On older subdivided orchard ground, irrigation infrastructure often survives the subdivision and runs along property lines with houses on both sides.
• Cover and drain window wells. A window well without a drain is a bucket set against a basement wall.

Where this leaves the structural question

Drainage correction generally runs $1,200 to $5,000, against $9,000 to $35,000 for a typical piering job. On houses with early hairline cracking and an obvious water source, fixing the water and watching a full annual cycle is a legitimate first step, and it settles the question more cheaply than anything else. Where movement has already progressed, the water work does not replace the structural repair, but it is what makes the structural repair last.

The drainage and grading page covers the scope, and the soils guide covers what the water is acting on.