The larger the lake, the most stable the flow out of it. Also, it helps that the lake be in a temperate environment, oo water input into the lake doesn't vary too much. The lake can freeze over, as can the outlet river, without really affecting flow much; this happens in the case of the example.
Example - Saint MaryMary's River (between Lake Superior and Lake Huron in US/Canada) at Sault Ste. Marie. Other good examples are the Rhone downstream of Lake Geneva, and the Neva downstream of Lake Ladoga.
Pro - Very stable flow rate, year round (assuming the lakes don't freeze). Low ratio and magnitude of seasonal fluctuations. When improved with canals and locks, this makes for a great trading city.
Con - In many cases, these outlet rivers are steep and un-navigable.
Rivers with stable flow (with data source)
River downstream of a large lakes
There are two large lakes. There is a river connecting them, probably short. Your city is on that river.
Example - Saint Mary River (between Lake Superior and Lake Huron in US/Canada) at Sault Ste. Marie.
Pro - Very stable flow rate, year round (assuming the lakes don't freeze). Low ratio and magnitude of seasonal fluctuations
Con - The only examples of these rivers in the real world are short, and they are generally steep and un-navigable. When improved with canals and locks, this makes for a great trading city.
Extreme high flow 3590
Expected annual high 2385
Average flow 2142
Excess over expect high 1205
Excess over average 1448
Ratio over expect high 0.51
Ratio over average 0.68
(Note, all units are in cubic meters per second, except ratio)
Large tropical river
A large tropical river where half of the river basin is in each hemisphere. As the monsoon rains are pushed by the Intertropical Convergence Zone back and forth between the hemispheres, you will end up with relatively stable rainfall throughout the year.
Example - Congo River measured at Kinshasa.
Pros - Very low seasonal flow variance. Also, an enormous basin for river-borne trade with the city.
Cons - Very high magnitude of flow variance. When the river is this big, even a little bit of flooding is a big deal.
Extreme high flow 80832
Expected annual high 56081
Average flow 39536
Excess over expect high 24751
Excess over average 41296
Ratio over expect high 0.44
Ratio over average 1.04
A temperate river with little snowmelt
The next best scenario is a river with a small basin with constant, low level rainfall, and little snow accumulation.
Example - Seine River at Paris.
Pro - Despite being a big enough river for a truly large city, the magnitude of highest recorded flood to average winter high water is not large.
Con - Large seasonal fluctuations, can be difficult to navigate in summer time due to low water levels.
Extreme high flow 1284
Expected annual high 560
Average flow 268
Excess over expect high 724
Excess over average 1016
Ratio over expect high 1.29
Ratio over average 3.79
Examples of bad rivers
Continental river with large, dry basin
Example Arkansas River in the US, measured at Little Rock. In the interior US, random thunderstorms lasting for a few days can cause serious flooding, usually in spring or early summer.
Extreme high flow 8220
Expected annual high 2044
Average flow 1066
Excess over expect high 6176
Excess over average 7145
Ratio over expect high 3.02
Ratio over average 6.71
Continental river with enormous spring snowmelt
Example - Tom River measured at Tomsk, in Siberia. While the high river levels with snowmelt is relatively predictable, it is still very large compared to regular river levels.
Extreme high flow 7500
Expected annual high 4622
Average flow 1047
Excess over expect high 2878
Excess over average 6453
Ratio over expect high 0.62
Ratio over average 6.16
River in Monsoon area
Example - Vijayawada river in southern India measured at its mouth. While the timing of the monsoon is predictable, its magnitude is not. An erratic monsoon can produce spectacular flooding.
Extreme high flow 16555
Expected annual high 6266
Average flow 1642
Excess over expect high 10289
Excess over average 14913
Ratio over expect high 1.64
Ratio over average 9.08