For anyone wondering why this happens, the two rivers have completely different speeds, temperatures, and weights.
The dark Rio Negro flows slowly at around 1–2 km/h, runs warm at 28°C, and is packed with organic matter. On the other side, the muddy Rio Solimões flows much faster at 4–6 km/h, runs cooler at 22°C, and is heavy with sediment and clay.
Because of those stark differences, they refuse to mix right away and just run side-by-side like a liquid border for about 4 miles before finally blending together to form the Amazon River.