This significant loss of suitable spawning habitat is due to lack of spawning-sized gravel recruitment from lateral and upstream sources due to the installation of manmade barriers that block this gravel from passing naturally downstream. Without recruitment, high flow releases scour gravel from spawning beds, so that they gradually become smaller and smaller. The smaller pieces of gravel are more easily picked up and moved downstream, leaving behind larger pieces that the salmon are unable to move to bury their eggs in a process called redd construction. This further crowds the salmon into smaller and smaller suitable spawning beds, which increases the likelihood of redd superimposition. This is when spawners construct their redds, or pockets in the gravel to lay their eggs in, on top of preexisting redds, thereby killing or burying some of the eggs in the pre-existing redds. The EPA has also stated that the remaining gravel is "probably fairly highly embedded and therefore of reduced quality and unavailable to fish for spawning". The Temperature of the water can also affect the level of spawning. The best temperatures for spring and fall run Chinook salmon are between 42 °F and 57 °F. However, in 77% of the years studied by the San Joaquin River Restoration Program, water temperatures were elevated above 57 °F and abnormalities and egg morbidity rates increased.