In mountain ranges that experience tectonic uplift (mountain building), rivers steepen and erode into the bedrock at a rate similar to the uplift rate. Without uplift, river erosion depends on sea level changes, rock type, how much water and sediment are in the channel, and the size of the sediment. On the Hawaiian Islands, there are deep canyons filled with boulders that are no longer eroding into bedrock, but some rivers are steeper than others. Why is that? The bedrock is all basalt, there is no tectonic uplift, and sea level has not changed significantly over time, so we can focus on the effects of water and sediment on river erosion. We hypothesized that rivers stopped eroding into the bedrock because they are filled with large boulders and/or sediment that the water is unable to move. To test this, we gathered erosion rate and grain size data on Kauai and West Maui, on the rainy and dry sides of each island. We found that river channel steepness normalized for basin area and rainfall correlates with both grain size and erosion rate, suggesting that boulders and sediment both influence river erosion on the islands.