Demographic History

I was exploring old Census Data and constructed the following graphs and tables. All data is for the municipalities and not the townships.

LaSalle peaks in 1930 with 13,149 residents.
Peru peaks in 1970 with 11,772 residents.
The greatest total population of the combined LaSalle-Peru was in 1970 with 22,508 residents. 

I've highlighted all growth rates greater than 10% in red and all growth rates less than zero in yellow; completely arbitrary colours that might have been better chosen.

Population Percent Change By Year

Boom: Between 1870 and 1880, LaSalle gained 51% more residents, while Peru gained 27% more residents. Growth rates continued to be high up through 1920. The area is heavily industrialized and expanding at more than modest rates. 
Bust: After 1920 LaSalle-Peru's growth rates seriously depart from the national growth rate.
Bust: Between 1930 to 1950, growth rates slowed and then went in the red (yellow in this case). LaSalle-Peru lost 2% and then 5% of its residents with LaSalle suffering the most. This happens to coincide with the time of the Great Depression...perhaps we could call it the "First Illinois Valley Depression"--Why not have a local economic history with locally themed names?
Boom: Between 1950 and 1970, Peru gained 21% and then 13% more residents. Reason unknown?
Bust: Between 1950 to 1970 LaSalle declines at 2% and then an alarming 10%. Perhaps this is due to the decline of M&H Zinco Co. Starting in 1961, various part of the Matthiessen and Hegeler Zinc Company were shutting down until the entire plant was closed in 1978. I dub this the "Zinc Recession."
Bust: Between 1970 and 1990, LaSalle-Peru lost 6% and then 13% of their residents. Westclox moved to Georgia on March 31st 1980. Since it employed greater than four thousand employees, the local economy could not absorb that many people in a short time span, so most left: the "Second Illinois Valley Depression."

Thankfully, between 1990 and 2000, LaSalle and Peru managed to pull themselves out of negative growth rates to grow at 4% and 6% respectively. Could this be the beginning of a Renaissance? I think it should be a goal to get up over 10% to be closer to the growth of the nation as a whole.

However, the census tract data show that 2010's population growth happened in the suburbs north of Peru and LaSalle's suburbs out near Utica. The rest of the town actually decreased in population, Downtown LaSalle losing about 5% and north LaSalle and Peru losing about 2%. In LaSalle, beneath 11th Street, over 10% of the housing units are vacant. Even then, I'd say the double dipped "Illinois Valley Depressions" are letting up and hope is on the way.

*the comments in this post are heavily based on the assumption that population growth is correlated with economic health. It sounds a reasonable assumption to me...