On the opposite side of the Mainstem Tuolumne from the Clavey River Confluence there is a bedrock formation that has been exposed by erosion and sediment transport in the streambed. This outcropping resembles a reef in the ocean. When the water flows over or around this reef, it creates a flow separation horizontally due to the lateral contraction of the stream at lower flow rates and vertically due to the recirculation zone created by the streamlines moving over the rocks at higher stage and velocity. In these flow separations, the stream velocity is much less than in the main part of the stream and is sometimes an eddy flowing in the opposite direction. As a result, sediment can be deposited behind these bedrock outcroppings that build up and create bars. The expansion bar that can be seen at lower water levels opposite the Clavey Confluence has been formed in this way.
Downstream at the North Fork Tuolumne Confluence, a similar thing has happened with the bedrock reef at the mouth of the river. In this case, some sediment is trapped behind the reef from the Northfork preventing it from reaching the mainstem. When the North Fork is higher it can deposit material onto the tributary bar just downstream of its confluence. The large, fairly uniform gravel section found near the North Fork Tuolumne was deposited by an eddy created by the Mainstem Tuolumne slowing down through the bedrock reef and the flow separation of the tributary meeting the mainstem. This gravel deposit is found on the second tier of tributary bar suggesting that it is only refreshed or covered in large flows (probably greater than 5000 cfs). On the lower tier of the tributary bar the mainstem is able to eat away at some of the tributary bar created by the Mainstem Tuolumne once the stage of the mainstem is high enough to not conflict with the bedrock formation at the mouth of the river.