Is a fabric-formed concrete mattress a good DIY option for creek erosion?

Those systems are usually called fabric-formed concrete mattresses, and they're definitly not using standard concrete. You need a high-strength **grout mix**, usually consisting of portland cement, sand, and water, sometimes with a bit of fly ash. You can't use pea gravel or any large aggregate because it'll bridge inside the fabric layers and cause a blowout or leave huge hollow spots. The mix needs to be fluid enough to flow like thick pancake batter so it fills every pocket of the mattress evenly. The fabric itself is a heavy-duty geotextile designed to handle UV exposure, but it's relly just a flexible form. Most of the longevity comes from the mass of the concrete blocks it creates. Even if the fabric eventually frays or wears away after fifteen years, you're left with an articulated system of blocks that have already settled into the shape of your slope. The secret to keeping it from sliding into the water is the header trench. You have to dig a deep trench at the very top of the hill, tuck the fabric in, and pin it down with rebar or heavy fill before you even think about calling the pump truck.

I've seen people try this as a DIY, and it usually goes sideways during the pour. You need a grout pump, not a standard boom pump, and you have to fill it from the bottom up so the air escapes. If you start at the top, the weight of the wet grout will just bulge the bottom and potentially rip the fabric. It’s a fast process once you start, but the site prep is everything.

One thing to keep in mind is that these mats are notorious for getting incredibly slippery. Since they're designed to be submerged or handle heavy rain, they grow algae and moss in the little valleys between the "ice cubes." If you’re planning on walking down to the stream, you’ll need to build in some actual stairs or a diferent path. It's great for stopping the dirt from washing away, but it’s not exactly a pedestrian-friendly surface.

The grout pump detail is a big help becuase I was honestly picturing a bunch of us with buckets and a regular mixer. Is that something a local rental yard even carries, or do you usally have to hire a specialist for that part?

Also, since it forms such a solid layer, where does the groundwater go? I’m worried that if I seal the hill, the pressure from a big rain might just push the whole concrete blanket right off the slope. Do people usually poke weep holes in the fabric for drainage, or does the grout breathe enough on its own to let the moisture through?

You're right to worry about the water pressure. Most of these mattresses have "filter points"—basicly spots where the fabric layers are joined so no grout fills there. It lets the hill breathe. Without those, the whole slab would just pop off the bank like a scab. If your yard is really wet, you might even want to lay down some drainage gravel or a French drain behind the header trench before you lay the fabric. It's alot of prep, but it beats watching your hard work slide into the creek becuase you turned your hill into a dam.

Check your local environmental regs before you pull the trigger. Pumping wet grout that close to a stream usually triggers a bunch of red tape becuase any spill or runoff can spike the pH and kill everythign in the water. You'll likely need some serious silt fencing or a buffer to keep the inspectors happy. On the looks side, that white fabric stays bright for a while, but you can actually toss some iron oxide pigment into the grout. It makes it blend in way better than that raw gray look once the fabric starts to wear thin and show the concrete underneath.

Don't overlook the slump on your grout. If it’s too runny, it’ll bleed out of the fabric pores, but if it’s too thick, you’ll get "honeycombing" where the blocks don't fully round out. I usally aim for a 10 to 12-inch flow on a spread test.

Also, watch the toe of the slope. If the stream undercuts the bottom edge, the whole mattress can unzip. You need to bury that bottom edge deep or wrap it around an anchor pipe to keep the current from getting underneath. If water scours out the base, the weight of the concrete won't save it from sliding.