Spreading grounds are an important link in the Los Angeles area water management system. Much of the water in the Greater Los Angeles region is derived from groundwater - water that is stored in porous layers underground. Public demand for water in our Mediterranian climate zone stresses water resources such as rivers and groundwater and often more is used that is replaced naturally.
Another problem is the controls put on rivers. Humans tend to like rivers that stay put, don't flood, and have sufficient water year-round. No one consulted the rivers, however. A series of floods on the Los Angeles and San Gabriel Rivers in 1914, 1933, and 1938 spurred public officials to create much of the Los Angeles flood control system you see today. Structures such as dams, flood control basins, and river channelization solved that problem for the human population in the area but ended up causing a different set of problems.
Together with the San Gabriel Canyon spreading grounds to the east of this location, The Fish Canyon spreading grounds are the first in a series of groundwater recharge areas along the San Gabriel River. Spreading grounds are areas where water in the river is allowed to pond up and slowly percolate down to the water table, recharging the aquifer below. Here at Fish Canyon, several low dams have been constructed across the San Gabriel River. During periods of high peak flow, the dams hold back some of the water while allowing the rest to flow over the top and continue downstream. Then, when the rains have ceased, the water naturally sinks into the ground.
While this solution solves the problems of groundwater recharge and flood control, the dams hinder the downstream movement of sediment. This means that parts of the river downstream are sediment starved, causing increased erosion along our coasts.
Looking downstream (south) at the top of one of the dams. Note how the river valley opens up as it exits the mountains and enters the alluvial fan.
A view across the top of the dam, looking west.
Looking upstream (north) into San Gabriel Canyon. Note the "V" shape of the canyon in the mountains.
Side Channel
A small side channel parallels the bike trail just to the east of the bike trail before crossing under the trail and entering the spreading grounds. It's only 2 feet wide and less than a foot deep, and the discharge is considerably smaller than it is further upstream. Human activities have generated a lot of trash.
Sediment analysis! Here is the quadrant again. The squares inside are 10 cm on each side.
Let's take a look inside the area highlighted in yellow.
The clast here is long on one axis, and narrow along the other. The long axis is about 8 cm and the short about 3 cm. This would place it in the "small cobble" category. Larger clasts = more energy.
This clast is more equant (equal on all sides) but is smaller than clast #1, about 3 cm in diameter. This would make it a "coarse pebble".
Where's the clast? It's all mud here! Smaller clasts = less energy.
Now, look back at the image. You can see my reflection in the water, which means I'm standing on the bank of the channel. The larger clasts (1 & 2) are near the bank, while the mud is closer to the center of the channel. Does this mean that there is more energy at the bank, and less towards the center of the river? No. Actually, it's the opposite - the water speed, and therefore the energy, is greater at the center of the river. So, what's going on? The water flow here is low, allowing for the sediments to settle out of the water and coat the riverbed with mud. There are larger clasts at the center of the channel - they are just covered in mud!
