If they're in the wash, especially the graded part - that is probably OK; it's Flood Control land. They aren't crazy about people being in flood channels, but they don't take any strong measures against pedestrian traffic. Just watch for flash floods, and don't drive in, even if the gate is open.
On National Forest, Wilderness, or National Park/Monument land, no natural materials are to be removed. Seeds, flowers, rocks, or anything. Exception being wood removed by registered permit, or purchased souvenirs at a ranger station or visitor center. At least this is my understanding.