Building a Stock Tank: Damming a Draw vs Digging a Pit

The watershed is the first question, not the last

Before anything else, somebody has to work out how many acres drain to the site. It is the number that decides whether a dammed tank fills, how big the spillway has to be, and frankly whether the site is a site at all. It gets skipped constantly, and skipping it produces both of the classic failures: a tank that never fills because there is not enough ground above it, and a dam that blows out because there is far more.

In caliche country the ground sheds hard. Very little soaks in, so a given acreage delivers more runoff here than the same acreage would somewhere with deeper soil and better cover. That cuts both ways — it is why modest draws out here can fill a good tank, and why a dam sized off a guess gets overtopped by a storm nobody thought was that big.

The spillway is what keeps the dam

Water going over the top of an earthen dam is how earthen dams are lost, and it does not take long. The emergency spillway exists so that when the tank fills and keeps filling, the overflow leaves through a wide, flat, grassed channel cut into undisturbed ground beside the dam, at an elevation below the top of the embankment, instead of over the structure itself.

Three things get it wrong. Cutting it too narrow, so it concentrates flow and cuts a channel. Setting it too high, leaving no freeboard between spillway level and the top of the dam. And cutting it through fill instead of into natural ground, which just gives the water a softer path to take the dam apart. If the budget forces a choice between a bigger tank and a proper spillway, build the smaller tank.

Key the dam in, or it leaks under itself

A dam built by piling dirt on top of the existing ground surface will seep underneath it, because the surface layer it is sitting on is loose, root-filled and permeable. The fix is a cutoff trench, a core trench cut down the centerline of the dam into tight material below, then backfilled with clay and compacted in lifts as the embankment goes up.

That trench is invisible in the finished tank and it is most of why one dam holds and the one across the fence does not. Same for the embankment itself: built in thin lifts with moisture in the material and rolled, not pushed up in a pile and shaped. There is no way to inspect this after the fact, which is why it is worth knowing whether it was done.

Depth beats width out here

Evaporation is the quiet thief in West Texas, and it works on surface area. A wide shallow tank and a narrower deep tank holding the same water at the start of summer are not in the same condition by August — the shallow one has more surface exposed for every acre-foot it holds, and it loses proportionally more.

Shallow edges cost you twice. They evaporate fastest, they grow the most vegetation, and they are where livestock churn the bank into mud. So tanks generally get built deeper than people first picture, with the banks pulled in steeper than a gentle slope and only the designated watering approach left shallow and armored.

Will the ground hold it

A tank holds what the soil lets it hold. Clay and tight material hold well. Sand, fractured rock or caliche with voids in it will take water down as fast as you can put it in, and no amount of good dam building fixes a bottom that leaks.

The way to find out is to dig test holes at the site before committing, not after. If the material is marginal there are answers — a clay liner hauled in and compacted, or a synthetic liner — and choosing between those is a whole decision of its own, which is the subject of our guide on dirt tanks versus sealed ponds. What you do not want is to find out by watching a full tank disappear over a month.

What we need to quote it

A drop pin on the site, or two or three if you are deciding between them. Roughly what drains to it. What the tank is for — livestock, wildlife, irrigation storage, fire protection — because it changes depth and shape. Whether there is a well or pipeline nearby. Access for equipment. And anything you know about the dirt, including any test holes or old tanks on the place that hold or do not.

We would rather walk it. Where the water already runs, what the banks are made of, and what old tanks on the same ground have done tell you more in twenty minutes than anything you can describe on the phone. Estimates are free, and if we think a site will not hold, we will say so before you spend money on it.

Frequently Asked Questions