When glacial ice melts, the water moves away from the glacial snout in fast-flowing streams and rivers. The water transports vast quantities of sediment and larger debris. If the sediment load is very large in relation to the velocity of the stream, the more coarse material may start to block the stream, choking it and forcing it to constantly change its course. The stream starts to diverge, splitting into numerous segments which split and join repeatedly. Braided streams are typically shallow and wide, surrounded by poorly sorted rock debris
http://www.geography-site.co.uk/pages/physical/glaciers/braided.html
http://pages.uoregon.edu/millerm/braided.html
Source:
Alaska ShoreZone
Braided river channels are variable and dynamic. Storm-driven waves erode them. Surging and retreating glaciers then deposit sediments, building them up. During periods of relative stability, opportunistic animals such as the blue mussel (Mytilus trossulus) and marine algae such as the rockweed (Fucus distichus) may colonize the beach face, creating the beautiful colored patterns seen here
https://toolkit.climate.gov/image/1231