What does the underlined mean? Did she use dirt or not? If she didn't, doesn't it have to be "because dir would wash away"?
42)Clara began to sketch a map of the escape route in the dirt of the fields. (c) She soon realized that although dirt would wash away, she could create a map in fabric and it would remain. She began collecting scraps of fabric as she sewed. The work took a long time; months could go by while (d) she waited for a scrap of cloth in the right color: blue for creeks and rivers, green for fields, white for roads. The more information Clara received, the larger her quilt became.
She started to sketch her map in the dirt but soon realised that the dirt might be washed away and therefore her map would be washed away at the same time. She also realised that if she made the map using pieces of fabric, she would be able to keep it. The fabric would not be destroyed as easily as the dirt. I agree with you that "although" was a slightly odd choice. I would simply have said "She soon realised that dirt might/would wash away but she could create a map in fabric and it would remain".