The damming of a river creates a reservoir upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam's construction. The newly created reservoir has more surface area than the river would have had, and therefore more evaporation occurs than previously. Rivers carry four different types of sediment down their riverbeds, allowing for the formation of riverbanks, river deltas, alluvial fans, braided rivers, oxbow lakes, levees and coastal shores. The construction of a dam blocks the flow of sediment downstream, leading to downstream erosion of these Sedimentary depositional environments, and increased sediment build-up in the reservoir. As all dams result in reduced sediment load downstream, a dammed river is said to be "hungry" for sediment. Because the rate of deposition of sediment is greatly reduced since there is less to deposit but the rate of erosion remains nearly constant, the water flow eats away at the river shores and riverbed, threatening shoreline ecosystems, deepening the riverbed, and narrowing the river over time. This leads to a compromised water table, reduced water levels, homogenization of the river flow and thus reduced ecosystem variability, reduced support for wildlife, and reduced amount of sediment reaching coastal plains and deltas. While the rate of sedimentation varies for each dam and each river, eventually all reservoirs develop a reduced water-storage capacity due to the exchange of storage space for sediment. This prompts coastal erosion, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. Channel erosion of rivers has its own set of consequences. The eroded channel could create a lower water table level in the affected area, impacting lowland crops.
A weir dam is a small overflow-type dam commonly used to raise the level of a river or stream. Weir dams have historically been used to create mill ponds but have also been used to create fishing ponds or to slow the rate of erosion. Water flows over the top of a weir. Since the weir is the portion where water is overflowing, a long weir allows a lot more water with a small increase in overflow depth. This is done in order to minimize fluctuation in the depth of the river upstream with changes in the flow rate of the river. Weirs also give hydrologists and engineers a simple method of measuring the rate of flow over the weir. There are different types of weirs. It may be a simple metal plate with a V notch cut into it or it may be a concrete and steel structure across the bed of a river.
This is an example of a Broad crested weir. They are large structures that are generally constructed from reinforced concrete and which usually span the full width of the river. 
info source: wikipedia
1. What is the width of the dam?
2. Does the dam go from bank to bank? Why is it that way?
3. What is the estimated flow rate of the water under the bridge?Does the dam alter the flow rate? Does it stop the erosion of the banks next to the dam and why?
4. (optional) Bonus post a picture of you on the bridge!
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