Earth Fissures in the Willcox Basin: How to Check Before You Build

In August 2021, after a heavy monsoon, the Arizona Geological Survey went out to look at two fissures in Cochise County. Water had flooded the existing cracks and eroded them rapidly. At Dragoon Road, west of US 191, the fissure measured about ten feet wide and thirteen feet deep. On Parker Ranch Road, east of Kansas Settlement Road, one had reached about sixteen and a half feet deep.

These are not geological curiosities happening somewhere else. They are on named roads in the middle of the area where most of the cheap land in this county is sold.

The map showing where they are is public, free, and takes two minutes to check. Very few people buying land here have looked at it.

What an earth fissure is

Not an earthquake fault. A fissure is a crack that opens on a valley floor when the ground subsides from long-term groundwater withdrawal.

The mechanism is straightforward. Sediments in a basin hold water in the pore space between grains. Pump the water out faster than it is replaced and that pore space compacts, and the land surface drops. Because the underlying bedrock is not flat, some parts of the basin drop more than others, and the ground gets pulled apart at the transitions. It cracks.

The crack usually starts underground and works upward, which is why the first sign at the surface is often a line of small collapse holes rather than an obvious open trench. Then rain gets into it. Surface water pours down the crack, erodes it from the inside, and what was a hairline becomes a gully — sometimes in a single storm season.

That last part is why fissures here are not a static risk. A parcel that looks fine can change during one monsoon.

The numbers for this basin

ADWR's own figures, from the material supporting the Willcox Active Management Area:

• More than 50 miles of earth fissuring recorded in the basin as of 2023.
• More than 11 feet of land subsidence, with the rate accelerating — 1.2 inches per year in 1996, 2.8 inches per year in 2023.
• Annual groundwater overdraft over 100,000 acre-feet.
• 71 wells gone dry in the last decade.

Longer-run context: groundwater levels in parts of the basin have dropped as much as 400 feet between the 1940s and 2015, and about five feet of subsidence had already been measured northwest of Willcox between 1937 and 1974. This has been happening for a long time and it is speeding up, not slowing down.

The state has been mapping it since 2006, when the legislature directed AZGS to establish an earth fissure mapping and monitoring program. Fissures crop out in five Arizona counties. Cochise is one of them.

How to check your parcel — five minutes

1. Open the AZGS Natural Hazards in Arizona Viewer. It carries the most current published distribution of Arizona earth fissures.
2. Find your parcel. Zoom to the area and cross-reference against the county GIS or a known landmark. Mapped fissure traces show as lines.
3. Look at the study area maps for your neighbourhood. AZGS publishes named fissure maps by area; the Cochise County set includes Dragoon Road, Three Sisters Buttes, North Sulphur Springs Valley and Croton Springs.
4. Check the surroundings, not just the outline. A mapped fissure a quarter mile away still matters. Fissures lengthen and new ones open along the same trends.
5. Then go and walk it. The map is a survey of what has been found and mapped, not a guarantee of what exists.

What to look for on the ground

Walking a parcel with fissures in mind, in the order they tend to appear:

• Lines of small sinkholes or collapse pits, a few feet across, roughly aligned. This is often the earliest surface expression.
• A hairline crack running straight for a long distance across otherwise undisturbed ground — straightness is the tell, since erosion gullies wander and fissures do not.
• Vegetation lined up unusually, where a crack has concentrated water and plants have followed it.
• A gully that starts and ends nowhere, not connected to a drainage above or below it.
• Fence lines or road surfaces stepped or offset along a line.
• Ponded water after rain in a linear pattern rather than in a natural low.

Walk it after a monsoon rain if you can. Fissures show themselves in wet ground far better than in dry.

What it means if there is one

It is not automatically fatal to the parcel, and the honest answer is that it depends where.

Do not build on or across one. A structure spanning a fissure is a structure that will be pulled apart, and no foundation design solves that at a residential budget.

Keep a real setback. Fissures widen at the surface through erosion, so the edge you see today is not the edge in five years. Ask AZGS or a geotechnical engineer what separation is appropriate for the specific feature rather than picking a number.

Keep surface water away from it. This is the part that is within your control and the part most directly connected to site work. Water running into a fissure is what turns a crack into a chasm. Grading that concentrates runoff toward a fissure will accelerate it — which means the drainage design on a parcel with a fissure is a different design.

Do not use it as a drainage outlet. It looks like a convenient place for water to go. It is a direct pathway to the aquifer, it will erode catastrophically, and it can contaminate the groundwater you or your neighbours drink.

Site the well and septic with it in mind. A fissure is a preferential flow path from the surface to depth. A drainfield draining toward one is a contamination route.

The state already treats this as a material fact

Where six or more parcels under 36 acres are sold as an unsubdivided subdivision in Arizona, the required disclosure report includes a soils report addressing subsidence, expansion and ground fissures — alongside flood risk and legal access.

That is worth sitting with. The state requires fissure disclosure on land sales because fissures are a known risk to land buyers in this state. If the transaction you are looking at does not trigger a public report, that risk has not gone away; the disclosure requirement has.

Where this connects to the well question

The fissures and the wells are the same story told from two ends. The basin subsides because it is being pumped faster than it recharges. That is the same overdraft that drove the median depth of new domestic wells in the Sunsites and Pearce area from 300 feet before 1990 to 540 feet for wells drilled since 2015, and the same overdraft behind the Active Management Area designation in January 2025.

If you are evaluating a parcel here, the fissure map and the well registry are two views of one underlying condition, and you should look at both on the same afternoon.

What we do about it

We are not geologists and this article is not a geotechnical assessment. What we do is site work, and on a parcel with a mapped or visible fissure that means designing the grading and drainage so runoff is directed away from the feature rather than into it, keeping disturbance back from it, and building the pad and driveway on the assumption that water will find any crack you give it.

If you have found something on your parcel and you are not sure whether it is a fissure or an erosion gully, photograph it, note where it is, and get AZGS or a geotechnical engineer to look. And before you site anything, check the map. It is free, it takes two minutes, and it is the single cheapest piece of diligence available on land in this basin.